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<h2>
declarative
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<li><a class="reference internal" href="#">declarative</a><ul>
<li><a class="reference internal" href="#synopsis">Synopsis</a></li>
<li><a class="reference internal" href="#defining-attributes">Defining Attributes</a></li>
<li><a class="reference internal" href="#association-of-metadata-and-engine">Association of Metadata and Engine</a></li>
<li><a class="reference internal" href="#configuring-relationships">Configuring Relationships</a></li>
<li><a class="reference internal" href="#configuring-many-to-many-relationships">Configuring Many-to-Many Relationships</a></li>
<li><a class="reference internal" href="#defining-synonyms">Defining Synonyms</a></li>
<li><a class="reference internal" href="#table-configuration">Table Configuration</a></li>
<li><a class="reference internal" href="#mapper-configuration">Mapper Configuration</a></li>
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<div class="section" id="module-sqlalchemy.ext.declarative">
<span id="declarative"></span><h1>declarative<a class="headerlink" href="#module-sqlalchemy.ext.declarative" title="Permalink to this headline">¶</a></h1>
<div class="section" id="synopsis">
<h2>Synopsis<a class="headerlink" href="#synopsis" title="Permalink to this headline">¶</a></h2>
<p>SQLAlchemy object-relational configuration involves the use of
<a class="reference internal" href="../sqlalchemy/schema.html#sqlalchemy.schema.Table" title="sqlalchemy.schema.Table"><tt class="xref py py-class docutils literal"><span class="pre">Table</span></tt></a>, <a class="reference internal" href="../orm/mapping.html#sqlalchemy.orm.mapper" title="sqlalchemy.orm.mapper"><tt class="xref py py-func docutils literal"><span class="pre">mapper()</span></tt></a>, and
class objects to define the three areas of configuration.
<a class="reference internal" href="#module-sqlalchemy.ext.declarative"><tt class="xref py py-mod docutils literal"><span class="pre">declarative</span></tt></a> allows all three types of
configuration to be expressed declaratively on an individual
mapped class. Regular SQLAlchemy schema elements and ORM constructs
are used in most cases.</p>
<p>As a simple example:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">sqlalchemy.ext.declarative</span> <span class="kn">import</span> <span class="n">declarative_base</span>
<span class="n">Base</span> <span class="o">=</span> <span class="n">declarative_base</span><span class="p">()</span>
<span class="k">class</span> <span class="nc">SomeClass</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'some_table'</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="n">name</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">50</span><span class="p">))</span></pre></div>
</div>
<p>Above, the <a class="reference internal" href="#sqlalchemy.ext.declarative.declarative_base" title="sqlalchemy.ext.declarative.declarative_base"><tt class="xref py py-func docutils literal"><span class="pre">declarative_base()</span></tt></a> callable returns a new base class from
which all mapped classes should inherit. When the class definition is
completed, a new <a class="reference internal" href="../sqlalchemy/schema.html#sqlalchemy.schema.Table" title="sqlalchemy.schema.Table"><tt class="xref py py-class docutils literal"><span class="pre">Table</span></tt></a> and
<a class="reference internal" href="../orm/mapping.html#sqlalchemy.orm.mapper" title="sqlalchemy.orm.mapper"><tt class="xref py py-class docutils literal"><span class="pre">mapper</span></tt></a> will have been generated, accessible via the
<tt class="docutils literal"><span class="pre">__table__</span></tt> and <tt class="docutils literal"><span class="pre">__mapper__</span></tt> attributes on the <tt class="docutils literal"><span class="pre">SomeClass</span></tt> class.</p>
</div>
<div class="section" id="defining-attributes">
<h2>Defining Attributes<a class="headerlink" href="#defining-attributes" title="Permalink to this headline">¶</a></h2>
<p>In the above example, the <a class="reference internal" href="../sqlalchemy/schema.html#sqlalchemy.schema.Column" title="sqlalchemy.schema.Column"><tt class="xref py py-class docutils literal"><span class="pre">Column</span></tt></a> objects are
automatically named with the name of the attribute to which they are
assigned.</p>
<p>They can also be explicitly named, and that name does not have to be
the same as name assigned on the class.
The column will be assigned to the <a class="reference internal" href="../sqlalchemy/schema.html#sqlalchemy.schema.Table" title="sqlalchemy.schema.Table"><tt class="xref py py-class docutils literal"><span class="pre">Table</span></tt></a> using the
given name, and mapped to the class using the attribute name:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">SomeClass</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'some_table'</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="s">"some_table_id"</span><span class="p">,</span> <span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="n">name</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="s">"name"</span><span class="p">,</span> <span class="n">String</span><span class="p">(</span><span class="mi">50</span><span class="p">))</span></pre></div>
</div>
<p>Attributes may be added to the class after its construction, and they will be
added to the underlying <a class="reference internal" href="../sqlalchemy/schema.html#sqlalchemy.schema.Table" title="sqlalchemy.schema.Table"><tt class="xref py py-class docutils literal"><span class="pre">Table</span></tt></a> and
<a class="reference internal" href="../orm/mapping.html#sqlalchemy.orm.mapper" title="sqlalchemy.orm.mapper"><tt class="xref py py-func docutils literal"><span class="pre">mapper()</span></tt></a> definitions as appropriate:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">SomeClass</span><span class="o">.</span><span class="n">data</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="s">'data'</span><span class="p">,</span> <span class="n">Unicode</span><span class="p">)</span>
<span class="n">SomeClass</span><span class="o">.</span><span class="n">related</span> <span class="o">=</span> <span class="n">relationship</span><span class="p">(</span><span class="n">RelatedInfo</span><span class="p">)</span></pre></div>
</div>
<p>Classes which are mapped explicitly using
<a class="reference internal" href="../orm/mapping.html#sqlalchemy.orm.mapper" title="sqlalchemy.orm.mapper"><tt class="xref py py-func docutils literal"><span class="pre">mapper()</span></tt></a> can interact freely with declarative
classes.</p>
<p>It is recommended, though not required, that all tables
share the same underlying <a class="reference internal" href="../sqlalchemy/schema.html#sqlalchemy.schema.MetaData" title="sqlalchemy.schema.MetaData"><tt class="xref py py-class docutils literal"><span class="pre">MetaData</span></tt></a> object,
so that string-configured <a class="reference internal" href="../sqlalchemy/schema.html#sqlalchemy.schema.ForeignKey" title="sqlalchemy.schema.ForeignKey"><tt class="xref py py-class docutils literal"><span class="pre">ForeignKey</span></tt></a>
references can be resolved without issue.</p>
</div>
<div class="section" id="association-of-metadata-and-engine">
<h2>Association of Metadata and Engine<a class="headerlink" href="#association-of-metadata-and-engine" title="Permalink to this headline">¶</a></h2>
<p>The <a class="reference internal" href="#sqlalchemy.ext.declarative.declarative_base" title="sqlalchemy.ext.declarative.declarative_base"><tt class="xref py py-func docutils literal"><span class="pre">declarative_base()</span></tt></a> base class contains a
<a class="reference internal" href="../sqlalchemy/schema.html#sqlalchemy.schema.MetaData" title="sqlalchemy.schema.MetaData"><tt class="xref py py-class docutils literal"><span class="pre">MetaData</span></tt></a> object where newly
defined <a class="reference internal" href="../sqlalchemy/schema.html#sqlalchemy.schema.Table" title="sqlalchemy.schema.Table"><tt class="xref py py-class docutils literal"><span class="pre">Table</span></tt></a> objects are collected. This
is accessed via the <a class="reference internal" href="../sqlalchemy/schema.html#sqlalchemy.schema.MetaData" title="sqlalchemy.schema.MetaData"><tt class="xref py py-class docutils literal"><span class="pre">MetaData</span></tt></a> class level
accessor, so to create tables we can say:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">engine</span> <span class="o">=</span> <span class="n">create_engine</span><span class="p">(</span><span class="s">'sqlite://'</span><span class="p">)</span>
<span class="n">Base</span><span class="o">.</span><span class="n">metadata</span><span class="o">.</span><span class="n">create_all</span><span class="p">(</span><span class="n">engine</span><span class="p">)</span></pre></div>
</div>
<p>The <a class="reference internal" href="../sqlalchemy/connections.html#sqlalchemy.engine.base.Engine" title="sqlalchemy.engine.base.Engine"><tt class="xref py py-class docutils literal"><span class="pre">Engine</span></tt></a> created above may also be
directly associated with the declarative base class using the <tt class="docutils literal"><span class="pre">bind</span></tt>
keyword argument, where it will be associated with the underlying
<a class="reference internal" href="../sqlalchemy/schema.html#sqlalchemy.schema.MetaData" title="sqlalchemy.schema.MetaData"><tt class="xref py py-class docutils literal"><span class="pre">MetaData</span></tt></a> object and allow SQL operations
involving that metadata and its tables to make use of that engine
automatically:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">Base</span> <span class="o">=</span> <span class="n">declarative_base</span><span class="p">(</span><span class="n">bind</span><span class="o">=</span><span class="n">create_engine</span><span class="p">(</span><span class="s">'sqlite://'</span><span class="p">))</span></pre></div>
</div>
<p>Alternatively, by way of the normal
<a class="reference internal" href="../sqlalchemy/schema.html#sqlalchemy.schema.MetaData" title="sqlalchemy.schema.MetaData"><tt class="xref py py-class docutils literal"><span class="pre">MetaData</span></tt></a> behavior, the <tt class="docutils literal"><span class="pre">bind</span></tt> attribute
of the class level accessor can be assigned at any time as follows:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">Base</span><span class="o">.</span><span class="n">metadata</span><span class="o">.</span><span class="n">bind</span> <span class="o">=</span> <span class="n">create_engine</span><span class="p">(</span><span class="s">'sqlite://'</span><span class="p">)</span></pre></div>
</div>
<p>The <a class="reference internal" href="#sqlalchemy.ext.declarative.declarative_base" title="sqlalchemy.ext.declarative.declarative_base"><tt class="xref py py-func docutils literal"><span class="pre">declarative_base()</span></tt></a> can also receive a pre-created
<a class="reference internal" href="../sqlalchemy/schema.html#sqlalchemy.schema.MetaData" title="sqlalchemy.schema.MetaData"><tt class="xref py py-class docutils literal"><span class="pre">MetaData</span></tt></a> object, which allows a
declarative setup to be associated with an already
existing traditional collection of <a class="reference internal" href="../sqlalchemy/schema.html#sqlalchemy.schema.Table" title="sqlalchemy.schema.Table"><tt class="xref py py-class docutils literal"><span class="pre">Table</span></tt></a>
objects:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">mymetadata</span> <span class="o">=</span> <span class="n">MetaData</span><span class="p">()</span>
<span class="n">Base</span> <span class="o">=</span> <span class="n">declarative_base</span><span class="p">(</span><span class="n">metadata</span><span class="o">=</span><span class="n">mymetadata</span><span class="p">)</span></pre></div>
</div>
</div>
<div class="section" id="configuring-relationships">
<h2>Configuring Relationships<a class="headerlink" href="#configuring-relationships" title="Permalink to this headline">¶</a></h2>
<p>Relationships to other classes are done in the usual way, with the added
feature that the class specified to <a class="reference internal" href="../orm/mapping.html#sqlalchemy.orm.relationship" title="sqlalchemy.orm.relationship"><tt class="xref py py-func docutils literal"><span class="pre">relationship()</span></tt></a>
may be a string name. The “class registry” associated with <tt class="docutils literal"><span class="pre">Base</span></tt>
is used at mapper compilation time to resolve the name into the actual
class object, which is expected to have been defined once the mapper
configuration is used:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">User</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'users'</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="n">name</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">50</span><span class="p">))</span>
<span class="n">addresses</span> <span class="o">=</span> <span class="n">relationship</span><span class="p">(</span><span class="s">"Address"</span><span class="p">,</span> <span class="n">backref</span><span class="o">=</span><span class="s">"user"</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">Address</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'addresses'</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="n">email</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">50</span><span class="p">))</span>
<span class="n">user_id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">ForeignKey</span><span class="p">(</span><span class="s">'users.id'</span><span class="p">))</span></pre></div>
</div>
<p>Column constructs, since they are just that, are immediately usable,
as below where we define a primary join condition on the <tt class="docutils literal"><span class="pre">Address</span></tt>
class using them:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">Address</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'addresses'</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="n">email</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">50</span><span class="p">))</span>
<span class="n">user_id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">ForeignKey</span><span class="p">(</span><span class="s">'users.id'</span><span class="p">))</span>
<span class="n">user</span> <span class="o">=</span> <span class="n">relationship</span><span class="p">(</span><span class="n">User</span><span class="p">,</span> <span class="n">primaryjoin</span><span class="o">=</span><span class="n">user_id</span> <span class="o">==</span> <span class="n">User</span><span class="o">.</span><span class="n">id</span><span class="p">)</span></pre></div>
</div>
<p>In addition to the main argument for <a class="reference internal" href="../orm/mapping.html#sqlalchemy.orm.relationship" title="sqlalchemy.orm.relationship"><tt class="xref py py-func docutils literal"><span class="pre">relationship()</span></tt></a>,
other arguments which depend upon the columns present on an as-yet
undefined class may also be specified as strings. These strings are
evaluated as Python expressions. The full namespace available within
this evaluation includes all classes mapped for this declarative base,
as well as the contents of the <tt class="docutils literal"><span class="pre">sqlalchemy</span></tt> package, including
expression functions like <a class="reference internal" href="../sqlalchemy/expressions.html#sqlalchemy.sql.expression.desc" title="sqlalchemy.sql.expression.desc"><tt class="xref py py-func docutils literal"><span class="pre">desc()</span></tt></a> and
<a class="reference internal" href="../sqlalchemy/expressions.html#sqlalchemy.sql.expression.func" title="sqlalchemy.sql.expression.func"><tt class="xref py py-attr docutils literal"><span class="pre">func</span></tt></a>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">User</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="c"># ....</span>
<span class="n">addresses</span> <span class="o">=</span> <span class="n">relationship</span><span class="p">(</span><span class="s">"Address"</span><span class="p">,</span>
<span class="n">order_by</span><span class="o">=</span><span class="s">"desc(Address.email)"</span><span class="p">,</span>
<span class="n">primaryjoin</span><span class="o">=</span><span class="s">"Address.user_id==User.id"</span><span class="p">)</span></pre></div>
</div>
<p>As an alternative to string-based attributes, attributes may also be
defined after all classes have been created. Just add them to the target
class after the fact:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">User</span><span class="o">.</span><span class="n">addresses</span> <span class="o">=</span> <span class="n">relationship</span><span class="p">(</span><span class="n">Address</span><span class="p">,</span>
<span class="n">primaryjoin</span><span class="o">=</span><span class="n">Address</span><span class="o">.</span><span class="n">user_id</span><span class="o">==</span><span class="n">User</span><span class="o">.</span><span class="n">id</span><span class="p">)</span></pre></div>
</div>
</div>
<div class="section" id="configuring-many-to-many-relationships">
<h2>Configuring Many-to-Many Relationships<a class="headerlink" href="#configuring-many-to-many-relationships" title="Permalink to this headline">¶</a></h2>
<p>There’s nothing special about many-to-many with declarative. The
<tt class="docutils literal"><span class="pre">secondary</span></tt> argument to <a class="reference internal" href="../orm/mapping.html#sqlalchemy.orm.relationship" title="sqlalchemy.orm.relationship"><tt class="xref py py-func docutils literal"><span class="pre">relationship()</span></tt></a> still
requires a <a class="reference internal" href="../sqlalchemy/schema.html#sqlalchemy.schema.Table" title="sqlalchemy.schema.Table"><tt class="xref py py-class docutils literal"><span class="pre">Table</span></tt></a> object, not a declarative
class. The <a class="reference internal" href="../sqlalchemy/schema.html#sqlalchemy.schema.Table" title="sqlalchemy.schema.Table"><tt class="xref py py-class docutils literal"><span class="pre">Table</span></tt></a> should share the same
<a class="reference internal" href="../sqlalchemy/schema.html#sqlalchemy.schema.MetaData" title="sqlalchemy.schema.MetaData"><tt class="xref py py-class docutils literal"><span class="pre">MetaData</span></tt></a> object used by the declarative
base:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">keywords</span> <span class="o">=</span> <span class="n">Table</span><span class="p">(</span>
<span class="s">'keywords'</span><span class="p">,</span> <span class="n">Base</span><span class="o">.</span><span class="n">metadata</span><span class="p">,</span>
<span class="n">Column</span><span class="p">(</span><span class="s">'author_id'</span><span class="p">,</span> <span class="n">Integer</span><span class="p">,</span> <span class="n">ForeignKey</span><span class="p">(</span><span class="s">'authors.id'</span><span class="p">)),</span>
<span class="n">Column</span><span class="p">(</span><span class="s">'keyword_id'</span><span class="p">,</span> <span class="n">Integer</span><span class="p">,</span> <span class="n">ForeignKey</span><span class="p">(</span><span class="s">'keywords.id'</span><span class="p">))</span>
<span class="p">)</span>
<span class="k">class</span> <span class="nc">Author</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'authors'</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="n">keywords</span> <span class="o">=</span> <span class="n">relationship</span><span class="p">(</span><span class="s">"Keyword"</span><span class="p">,</span> <span class="n">secondary</span><span class="o">=</span><span class="n">keywords</span><span class="p">)</span></pre></div>
</div>
<p>You should generally <strong>not</strong> map a class and also specify its table in
a many-to-many relationship, since the ORM may issue duplicate INSERT and
DELETE statements.</p>
</div>
<div class="section" id="defining-synonyms">
<h2>Defining Synonyms<a class="headerlink" href="#defining-synonyms" title="Permalink to this headline">¶</a></h2>
<p>Synonyms are introduced in <a class="reference internal" href="../../mappers.html#synonyms"><em>Using Descriptors</em></a>. To define a getter/setter
which proxies to an underlying attribute, use
<a class="reference internal" href="../orm/mapping.html#sqlalchemy.orm.synonym" title="sqlalchemy.orm.synonym"><tt class="xref py py-func docutils literal"><span class="pre">synonym()</span></tt></a> with the <tt class="docutils literal"><span class="pre">descriptor</span></tt> argument:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">MyClass</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'sometable'</span>
<span class="n">_attr</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="s">'attr'</span><span class="p">,</span> <span class="n">String</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">_get_attr</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">_some_attr</span>
<span class="k">def</span> <span class="nf">_set_attr</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">attr</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">_some_attr</span> <span class="o">=</span> <span class="n">attr</span>
<span class="n">attr</span> <span class="o">=</span> <span class="n">synonym</span><span class="p">(</span><span class="s">'_attr'</span><span class="p">,</span> <span class="n">descriptor</span><span class="o">=</span><span class="nb">property</span><span class="p">(</span><span class="n">_get_attr</span><span class="p">,</span> <span class="n">_set_attr</span><span class="p">))</span></pre></div>
</div>
<p>The above synonym is then usable as an instance attribute as well as a
class-level expression construct:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">x</span> <span class="o">=</span> <span class="n">MyClass</span><span class="p">()</span>
<span class="n">x</span><span class="o">.</span><span class="n">attr</span> <span class="o">=</span> <span class="s">"some value"</span>
<span class="n">session</span><span class="o">.</span><span class="n">query</span><span class="p">(</span><span class="n">MyClass</span><span class="p">)</span><span class="o">.</span><span class="n">filter</span><span class="p">(</span><span class="n">MyClass</span><span class="o">.</span><span class="n">attr</span> <span class="o">==</span> <span class="s">'some other value'</span><span class="p">)</span><span class="o">.</span><span class="n">all</span><span class="p">()</span></pre></div>
</div>
<p>For simple getters, the <a class="reference internal" href="#sqlalchemy.ext.declarative.synonym_for" title="sqlalchemy.ext.declarative.synonym_for"><tt class="xref py py-func docutils literal"><span class="pre">synonym_for()</span></tt></a> decorator can be used in
conjunction with <tt class="docutils literal"><span class="pre">@property</span></tt>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">MyClass</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'sometable'</span>
<span class="n">_attr</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="s">'attr'</span><span class="p">,</span> <span class="n">String</span><span class="p">)</span>
<span class="nd">@synonym_for</span><span class="p">(</span><span class="s">'_attr'</span><span class="p">)</span>
<span class="nd">@property</span>
<span class="k">def</span> <span class="nf">attr</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">_some_attr</span></pre></div>
</div>
<p>Similarly, <a class="reference internal" href="#sqlalchemy.ext.declarative.comparable_using" title="sqlalchemy.ext.declarative.comparable_using"><tt class="xref py py-func docutils literal"><span class="pre">comparable_using()</span></tt></a> is a front end for the
<a class="reference internal" href="../orm/mapping.html#sqlalchemy.orm.comparable_property" title="sqlalchemy.orm.comparable_property"><tt class="xref py py-func docutils literal"><span class="pre">comparable_property()</span></tt></a> ORM function:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">MyClass</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'sometable'</span>
<span class="n">name</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="s">'name'</span><span class="p">,</span> <span class="n">String</span><span class="p">)</span>
<span class="nd">@comparable_using</span><span class="p">(</span><span class="n">MyUpperCaseComparator</span><span class="p">)</span>
<span class="nd">@property</span>
<span class="k">def</span> <span class="nf">uc_name</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">name</span><span class="o">.</span><span class="n">upper</span><span class="p">()</span></pre></div>
</div>
</div>
<div class="section" id="table-configuration">
<h2>Table Configuration<a class="headerlink" href="#table-configuration" title="Permalink to this headline">¶</a></h2>
<p>Table arguments other than the name, metadata, and mapped Column
arguments are specified using the <tt class="docutils literal"><span class="pre">__table_args__</span></tt> class attribute.
This attribute accommodates both positional as well as keyword
arguments that are normally sent to the
<a class="reference internal" href="../sqlalchemy/schema.html#sqlalchemy.schema.Table" title="sqlalchemy.schema.Table"><tt class="xref py py-class docutils literal"><span class="pre">Table</span></tt></a> constructor.
The attribute can be specified in one of two forms. One is as a
dictionary:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">MyClass</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'sometable'</span>
<span class="n">__table_args__</span> <span class="o">=</span> <span class="p">{</span><span class="s">'mysql_engine'</span><span class="p">:</span><span class="s">'InnoDB'</span><span class="p">}</span></pre></div>
</div>
<p>The other, a tuple of the form
<tt class="docutils literal"><span class="pre">(arg1,</span> <span class="pre">arg2,</span> <span class="pre">...,</span> <span class="pre">{kwarg1:value,</span> <span class="pre">...})</span></tt>, which allows positional
arguments to be specified as well (usually constraints):</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">MyClass</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'sometable'</span>
<span class="n">__table_args__</span> <span class="o">=</span> <span class="p">(</span>
<span class="n">ForeignKeyConstraint</span><span class="p">([</span><span class="s">'id'</span><span class="p">],</span> <span class="p">[</span><span class="s">'remote_table.id'</span><span class="p">]),</span>
<span class="n">UniqueConstraint</span><span class="p">(</span><span class="s">'foo'</span><span class="p">),</span>
<span class="p">{</span><span class="s">'autoload'</span><span class="p">:</span><span class="bp">True</span><span class="p">}</span>
<span class="p">)</span></pre></div>
</div>
<p>Note that the keyword parameters dictionary is required in the tuple
form even if empty.</p>
<p>As an alternative to <tt class="docutils literal"><span class="pre">__tablename__</span></tt>, a direct
<a class="reference internal" href="../sqlalchemy/schema.html#sqlalchemy.schema.Table" title="sqlalchemy.schema.Table"><tt class="xref py py-class docutils literal"><span class="pre">Table</span></tt></a> construct may be used. The
<a class="reference internal" href="../sqlalchemy/schema.html#sqlalchemy.schema.Column" title="sqlalchemy.schema.Column"><tt class="xref py py-class docutils literal"><span class="pre">Column</span></tt></a> objects, which in this case require
their names, will be added to the mapping just like a regular mapping
to a table:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">MyClass</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="n">__table__</span> <span class="o">=</span> <span class="n">Table</span><span class="p">(</span><span class="s">'my_table'</span><span class="p">,</span> <span class="n">Base</span><span class="o">.</span><span class="n">metadata</span><span class="p">,</span>
<span class="n">Column</span><span class="p">(</span><span class="s">'id'</span><span class="p">,</span> <span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">),</span>
<span class="n">Column</span><span class="p">(</span><span class="s">'name'</span><span class="p">,</span> <span class="n">String</span><span class="p">(</span><span class="mi">50</span><span class="p">))</span>
<span class="p">)</span></pre></div>
</div>
</div>
<div class="section" id="mapper-configuration">
<h2>Mapper Configuration<a class="headerlink" href="#mapper-configuration" title="Permalink to this headline">¶</a></h2>
<p>Configuration of mappers is done with the
<a class="reference internal" href="../orm/mapping.html#sqlalchemy.orm.mapper" title="sqlalchemy.orm.mapper"><tt class="xref py py-func docutils literal"><span class="pre">mapper()</span></tt></a> function and all the possible mapper
configuration parameters can be found in the documentation for that
function.</p>
<p><a class="reference internal" href="../orm/mapping.html#sqlalchemy.orm.mapper" title="sqlalchemy.orm.mapper"><tt class="xref py py-func docutils literal"><span class="pre">mapper()</span></tt></a> is still used by declaratively mapped
classes and keyword parameters to the function can be passed by
placing them in the <tt class="docutils literal"><span class="pre">__mapper_args__</span></tt> class variable:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">Widget</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'widgets'</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="n">__mapper_args__</span> <span class="o">=</span> <span class="p">{</span><span class="s">'extension'</span><span class="p">:</span> <span class="n">MyWidgetExtension</span><span class="p">()}</span></pre></div>
</div>
</div>
<div class="section" id="inheritance-configuration">
<h2>Inheritance Configuration<a class="headerlink" href="#inheritance-configuration" title="Permalink to this headline">¶</a></h2>
<p>Declarative supports all three forms of inheritance as intuitively
as possible. The <tt class="docutils literal"><span class="pre">inherits</span></tt> mapper keyword argument is not needed
as declarative will determine this from the class itself. The various
“polymorphic” keyword arguments are specified using <tt class="docutils literal"><span class="pre">__mapper_args__</span></tt>.</p>
<div class="section" id="joined-table-inheritance">
<h3>Joined Table Inheritance<a class="headerlink" href="#joined-table-inheritance" title="Permalink to this headline">¶</a></h3>
<p>Joined table inheritance is defined as a subclass that defines its own
table:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">Person</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'people'</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="n">discriminator</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="s">'type'</span><span class="p">,</span> <span class="n">String</span><span class="p">(</span><span class="mi">50</span><span class="p">))</span>
<span class="n">__mapper_args__</span> <span class="o">=</span> <span class="p">{</span><span class="s">'polymorphic_on'</span><span class="p">:</span> <span class="n">discriminator</span><span class="p">}</span>
<span class="k">class</span> <span class="nc">Engineer</span><span class="p">(</span><span class="n">Person</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'engineers'</span>
<span class="n">__mapper_args__</span> <span class="o">=</span> <span class="p">{</span><span class="s">'polymorphic_identity'</span><span class="p">:</span> <span class="s">'engineer'</span><span class="p">}</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">ForeignKey</span><span class="p">(</span><span class="s">'people.id'</span><span class="p">),</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="n">primary_language</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">50</span><span class="p">))</span></pre></div>
</div>
<p>Note that above, the <tt class="docutils literal"><span class="pre">Engineer.id</span></tt> attribute, since it shares the
same attribute name as the <tt class="docutils literal"><span class="pre">Person.id</span></tt> attribute, will in fact
represent the <tt class="docutils literal"><span class="pre">people.id</span></tt> and <tt class="docutils literal"><span class="pre">engineers.id</span></tt> columns together, and
will render inside a query as <tt class="docutils literal"><span class="pre">"people.id"</span></tt>.
To provide the <tt class="docutils literal"><span class="pre">Engineer</span></tt> class with an attribute that represents
only the <tt class="docutils literal"><span class="pre">engineers.id</span></tt> column, give it a different attribute name:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">Engineer</span><span class="p">(</span><span class="n">Person</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'engineers'</span>
<span class="n">__mapper_args__</span> <span class="o">=</span> <span class="p">{</span><span class="s">'polymorphic_identity'</span><span class="p">:</span> <span class="s">'engineer'</span><span class="p">}</span>
<span class="n">engineer_id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="s">'id'</span><span class="p">,</span> <span class="n">Integer</span><span class="p">,</span> <span class="n">ForeignKey</span><span class="p">(</span><span class="s">'people.id'</span><span class="p">),</span>
<span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="n">primary_language</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">50</span><span class="p">))</span></pre></div>
</div>
</div>
<div class="section" id="single-table-inheritance">
<h3>Single Table Inheritance<a class="headerlink" href="#single-table-inheritance" title="Permalink to this headline">¶</a></h3>
<p>Single table inheritance is defined as a subclass that does not have
its own table; you just leave out the <tt class="docutils literal"><span class="pre">__table__</span></tt> and <tt class="docutils literal"><span class="pre">__tablename__</span></tt>
attributes:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">Person</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'people'</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="n">discriminator</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="s">'type'</span><span class="p">,</span> <span class="n">String</span><span class="p">(</span><span class="mi">50</span><span class="p">))</span>
<span class="n">__mapper_args__</span> <span class="o">=</span> <span class="p">{</span><span class="s">'polymorphic_on'</span><span class="p">:</span> <span class="n">discriminator</span><span class="p">}</span>
<span class="k">class</span> <span class="nc">Engineer</span><span class="p">(</span><span class="n">Person</span><span class="p">):</span>
<span class="n">__mapper_args__</span> <span class="o">=</span> <span class="p">{</span><span class="s">'polymorphic_identity'</span><span class="p">:</span> <span class="s">'engineer'</span><span class="p">}</span>
<span class="n">primary_language</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">50</span><span class="p">))</span></pre></div>
</div>
<p>When the above mappers are configured, the <tt class="docutils literal"><span class="pre">Person</span></tt> class is mapped
to the <tt class="docutils literal"><span class="pre">people</span></tt> table <em>before</em> the <tt class="docutils literal"><span class="pre">primary_language</span></tt> column is
defined, and this column will not be included in its own mapping.
When <tt class="docutils literal"><span class="pre">Engineer</span></tt> then defines the <tt class="docutils literal"><span class="pre">primary_language</span></tt> column, the
column is added to the <tt class="docutils literal"><span class="pre">people</span></tt> table so that it is included in the
mapping for <tt class="docutils literal"><span class="pre">Engineer</span></tt> and is also part of the table’s full set of
columns. Columns which are not mapped to <tt class="docutils literal"><span class="pre">Person</span></tt> are also excluded
from any other single or joined inheriting classes using the
<tt class="docutils literal"><span class="pre">exclude_properties</span></tt> mapper argument. Below, <tt class="docutils literal"><span class="pre">Manager</span></tt> will have
all the attributes of <tt class="docutils literal"><span class="pre">Person</span></tt> and <tt class="docutils literal"><span class="pre">Manager</span></tt> but <em>not</em> the
<tt class="docutils literal"><span class="pre">primary_language</span></tt> attribute of <tt class="docutils literal"><span class="pre">Engineer</span></tt>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">Manager</span><span class="p">(</span><span class="n">Person</span><span class="p">):</span>
<span class="n">__mapper_args__</span> <span class="o">=</span> <span class="p">{</span><span class="s">'polymorphic_identity'</span><span class="p">:</span> <span class="s">'manager'</span><span class="p">}</span>
<span class="n">golf_swing</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">50</span><span class="p">))</span></pre></div>
</div>
<p>The attribute exclusion logic is provided by the
<tt class="docutils literal"><span class="pre">exclude_properties</span></tt> mapper argument, and declarative’s default
behavior can be disabled by passing an explicit <tt class="docutils literal"><span class="pre">exclude_properties</span></tt>
collection (empty or otherwise) to the <tt class="docutils literal"><span class="pre">__mapper_args__</span></tt>.</p>
</div>
<div class="section" id="concrete-table-inheritance">
<h3>Concrete Table Inheritance<a class="headerlink" href="#concrete-table-inheritance" title="Permalink to this headline">¶</a></h3>
<p>Concrete is defined as a subclass which has its own table and sets the
<tt class="docutils literal"><span class="pre">concrete</span></tt> keyword argument to <tt class="xref docutils literal"><span class="pre">True</span></tt>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">Person</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'people'</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="n">name</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">50</span><span class="p">))</span>
<span class="k">class</span> <span class="nc">Engineer</span><span class="p">(</span><span class="n">Person</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'engineers'</span>
<span class="n">__mapper_args__</span> <span class="o">=</span> <span class="p">{</span><span class="s">'concrete'</span><span class="p">:</span><span class="bp">True</span><span class="p">}</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="n">primary_language</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">50</span><span class="p">))</span>
<span class="n">name</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">50</span><span class="p">))</span></pre></div>
</div>
<p>Usage of an abstract base class is a little less straightforward as it
requires usage of <a class="reference internal" href="../orm/utilities.html#sqlalchemy.orm.util.polymorphic_union" title="sqlalchemy.orm.util.polymorphic_union"><tt class="xref py py-func docutils literal"><span class="pre">polymorphic_union()</span></tt></a>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">engineers</span> <span class="o">=</span> <span class="n">Table</span><span class="p">(</span><span class="s">'engineers'</span><span class="p">,</span> <span class="n">Base</span><span class="o">.</span><span class="n">metadata</span><span class="p">,</span>
<span class="n">Column</span><span class="p">(</span><span class="s">'id'</span><span class="p">,</span> <span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">),</span>
<span class="n">Column</span><span class="p">(</span><span class="s">'name'</span><span class="p">,</span> <span class="n">String</span><span class="p">(</span><span class="mi">50</span><span class="p">)),</span>
<span class="n">Column</span><span class="p">(</span><span class="s">'primary_language'</span><span class="p">,</span> <span class="n">String</span><span class="p">(</span><span class="mi">50</span><span class="p">))</span>
<span class="p">)</span>
<span class="n">managers</span> <span class="o">=</span> <span class="n">Table</span><span class="p">(</span><span class="s">'managers'</span><span class="p">,</span> <span class="n">Base</span><span class="o">.</span><span class="n">metadata</span><span class="p">,</span>
<span class="n">Column</span><span class="p">(</span><span class="s">'id'</span><span class="p">,</span> <span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">),</span>
<span class="n">Column</span><span class="p">(</span><span class="s">'name'</span><span class="p">,</span> <span class="n">String</span><span class="p">(</span><span class="mi">50</span><span class="p">)),</span>
<span class="n">Column</span><span class="p">(</span><span class="s">'golf_swing'</span><span class="p">,</span> <span class="n">String</span><span class="p">(</span><span class="mi">50</span><span class="p">))</span>
<span class="p">)</span>
<span class="n">punion</span> <span class="o">=</span> <span class="n">polymorphic_union</span><span class="p">({</span>
<span class="s">'engineer'</span><span class="p">:</span><span class="n">engineers</span><span class="p">,</span>
<span class="s">'manager'</span><span class="p">:</span><span class="n">managers</span>
<span class="p">},</span> <span class="s">'type'</span><span class="p">,</span> <span class="s">'punion'</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">Person</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="n">__table__</span> <span class="o">=</span> <span class="n">punion</span>
<span class="n">__mapper_args__</span> <span class="o">=</span> <span class="p">{</span><span class="s">'polymorphic_on'</span><span class="p">:</span><span class="n">punion</span><span class="o">.</span><span class="n">c</span><span class="o">.</span><span class="n">type</span><span class="p">}</span>
<span class="k">class</span> <span class="nc">Engineer</span><span class="p">(</span><span class="n">Person</span><span class="p">):</span>
<span class="n">__table__</span> <span class="o">=</span> <span class="n">engineers</span>
<span class="n">__mapper_args__</span> <span class="o">=</span> <span class="p">{</span><span class="s">'polymorphic_identity'</span><span class="p">:</span><span class="s">'engineer'</span><span class="p">,</span> <span class="s">'concrete'</span><span class="p">:</span><span class="bp">True</span><span class="p">}</span>
<span class="k">class</span> <span class="nc">Manager</span><span class="p">(</span><span class="n">Person</span><span class="p">):</span>
<span class="n">__table__</span> <span class="o">=</span> <span class="n">managers</span>
<span class="n">__mapper_args__</span> <span class="o">=</span> <span class="p">{</span><span class="s">'polymorphic_identity'</span><span class="p">:</span><span class="s">'manager'</span><span class="p">,</span> <span class="s">'concrete'</span><span class="p">:</span><span class="bp">True</span><span class="p">}</span></pre></div>
</div>
</div>
</div>
<div class="section" id="mixin-classes">
<h2>Mixin Classes<a class="headerlink" href="#mixin-classes" title="Permalink to this headline">¶</a></h2>
<p>A common need when using <a class="reference internal" href="#module-sqlalchemy.ext.declarative"><tt class="xref py py-mod docutils literal"><span class="pre">declarative</span></tt></a> is to
share some functionality, often a set of columns, across many
classes. The normal Python idiom would be to put this common code into
a base class and have all the other classes subclass this class.</p>
<p>When using <a class="reference internal" href="#module-sqlalchemy.ext.declarative"><tt class="xref py py-mod docutils literal"><span class="pre">declarative</span></tt></a>, this need is met by
using a “mixin class”. A mixin class is one that isn’t mapped to a
table and doesn’t subclass the declarative <tt class="xref py py-class docutils literal"><span class="pre">Base</span></tt>. For example:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">MyMixin</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="n">__table_args__</span> <span class="o">=</span> <span class="p">{</span><span class="s">'mysql_engine'</span><span class="p">:</span> <span class="s">'InnoDB'</span><span class="p">}</span>
<span class="n">__mapper_args__</span><span class="o">=</span> <span class="p">{</span><span class="s">'always_refresh'</span><span class="p">:</span> <span class="bp">True</span><span class="p">}</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">MyModel</span><span class="p">(</span><span class="n">Base</span><span class="p">,</span><span class="n">MyMixin</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'test'</span>
<span class="n">name</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">1000</span><span class="p">))</span></pre></div>
</div>
<p>Where above, the class <tt class="docutils literal"><span class="pre">MyModel</span></tt> will contain an “id” column
as well as <tt class="docutils literal"><span class="pre">__table_args__</span></tt> and <tt class="docutils literal"><span class="pre">__mapper_args__</span></tt> defined
by the <tt class="docutils literal"><span class="pre">MyMixin</span></tt> mixin class.</p>
<div class="section" id="mixing-in-columns">
<h3>Mixing in Columns<a class="headerlink" href="#mixing-in-columns" title="Permalink to this headline">¶</a></h3>
<p>The most basic way to specify a column on a mixin is by simple
declaration:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">TimestampMixin</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="n">created_at</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">DateTime</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="n">func</span><span class="o">.</span><span class="n">now</span><span class="p">())</span>
<span class="k">class</span> <span class="nc">MyModel</span><span class="p">(</span><span class="n">Base</span><span class="p">,</span> <span class="n">TimestampMixin</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'test'</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="n">name</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">1000</span><span class="p">))</span></pre></div>
</div>
<p>Where above, all declarative classes that include <tt class="docutils literal"><span class="pre">TimestampMixin</span></tt>
will also have a column <tt class="docutils literal"><span class="pre">created_at</span></tt> that applies a timestamp to
all row insertions.</p>
<p>Those familiar with the SQLAlchemy expression language know that
the object identity of clause elements defines their role in a schema.
Two <tt class="docutils literal"><span class="pre">Table</span></tt> objects <tt class="docutils literal"><span class="pre">a</span></tt> and <tt class="docutils literal"><span class="pre">b</span></tt> may both have a column called
<tt class="docutils literal"><span class="pre">id</span></tt>, but the way these are differentiated is that <tt class="docutils literal"><span class="pre">a.c.id</span></tt>
and <tt class="docutils literal"><span class="pre">b.c.id</span></tt> are two distinct Python objects, referencing their
parent tables <tt class="docutils literal"><span class="pre">a</span></tt> and <tt class="docutils literal"><span class="pre">b</span></tt> respectively.</p>
<p>In the case of the mixin column, it seems that only one
<tt class="xref py py-class docutils literal"><span class="pre">Column</span></tt> object is explicitly created, yet the ultimate
<tt class="docutils literal"><span class="pre">created_at</span></tt> column above must exist as a distinct Python object
for each separate destination class. To accomplish this, the declarative
extension creates a <strong>copy</strong> of each <tt class="xref py py-class docutils literal"><span class="pre">Column</span></tt> object encountered on
a class that is detected as a mixin.</p>
<p>This copy mechanism is limited to simple columns that have no foreign
keys, as a <tt class="xref py py-class docutils literal"><span class="pre">ForeignKey</span></tt> itself contains references to columns
which can’t be properly recreated at this level. For columns that
have foreign keys, as well as for the variety of mapper-level constructs
that require destination-explicit context, the
<a class="reference internal" href="../sqlalchemy/util.html#sqlalchemy.util.classproperty" title="sqlalchemy.util.classproperty"><tt class="xref py py-func docutils literal"><span class="pre">classproperty()</span></tt></a> decorator is provided so that
patterns common to many classes can be defined as callables:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">sqlalchemy.util</span> <span class="kn">import</span> <span class="n">classproperty</span>
<span class="k">class</span> <span class="nc">ReferenceAddressMixin</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="nd">@classproperty</span>
<span class="k">def</span> <span class="nf">address_id</span><span class="p">(</span><span class="n">cls</span><span class="p">):</span>
<span class="k">return</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">ForeignKey</span><span class="p">(</span><span class="s">'address.id'</span><span class="p">))</span>
<span class="k">class</span> <span class="nc">User</span><span class="p">(</span><span class="n">Base</span><span class="p">,</span> <span class="n">ReferenceAddressMixin</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'user'</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span></pre></div>
</div>
<p>Where above, the <tt class="docutils literal"><span class="pre">address_id</span></tt> class-level callable is executed at the
point at which the <tt class="docutils literal"><span class="pre">User</span></tt> class is constructed, and the declarative
extension can use the resulting <tt class="xref py py-class docutils literal"><span class="pre">Column</span></tt> object as returned by
the method without the need to copy it.</p>
<p>Columns generated by <a class="reference internal" href="../sqlalchemy/util.html#sqlalchemy.util.classproperty" title="sqlalchemy.util.classproperty"><tt class="xref py py-func docutils literal"><span class="pre">classproperty()</span></tt></a> can also be
referenced by <tt class="docutils literal"><span class="pre">__mapper_args__</span></tt> to a limited degree, currently
by <tt class="docutils literal"><span class="pre">polymorphic_on</span></tt> and <tt class="docutils literal"><span class="pre">version_id_col</span></tt>, by specifying the
classdecorator itself into the dictionary - the declarative extension
will resolve them at class construction time:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">MyMixin</span><span class="p">:</span>
<span class="nd">@classproperty</span>
<span class="k">def</span> <span class="nf">type_</span><span class="p">(</span><span class="n">cls</span><span class="p">):</span>
<span class="k">return</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">50</span><span class="p">))</span>
<span class="n">__mapper_args__</span><span class="o">=</span> <span class="p">{</span><span class="s">'polymorphic_on'</span><span class="p">:</span><span class="n">type_</span><span class="p">}</span>
<span class="k">class</span> <span class="nc">MyModel</span><span class="p">(</span><span class="n">Base</span><span class="p">,</span><span class="n">MyMixin</span><span class="p">):</span>
<span class="n">__tablename__</span><span class="o">=</span><span class="s">'test'</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span></pre></div>
</div>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">The usage of <a class="reference internal" href="../sqlalchemy/util.html#sqlalchemy.util.classproperty" title="sqlalchemy.util.classproperty"><tt class="xref py py-func docutils literal"><span class="pre">classproperty()</span></tt></a> with mixin
columns is a new feature as of SQLAlchemy 0.6.2.</p>
</div>
</div>
<div class="section" id="mixing-in-relationships">
<h3>Mixing in Relationships<a class="headerlink" href="#mixing-in-relationships" title="Permalink to this headline">¶</a></h3>
<p>Relationships created by <a class="reference internal" href="../orm/mapping.html#sqlalchemy.orm.relationship" title="sqlalchemy.orm.relationship"><tt class="xref py py-func docutils literal"><span class="pre">relationship()</span></tt></a> are provided
exclusively using the <a class="reference internal" href="../sqlalchemy/util.html#sqlalchemy.util.classproperty" title="sqlalchemy.util.classproperty"><tt class="xref py py-func docutils literal"><span class="pre">classproperty()</span></tt></a> approach,
eliminating any ambiguity which could arise when copying a relationship
and its possibly column-bound contents. Below is an example which
combines a foreign key column and a relationship so that two classes
<tt class="docutils literal"><span class="pre">Foo</span></tt> and <tt class="docutils literal"><span class="pre">Bar</span></tt> can both be configured to reference a common
target class via many-to-one:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">RefTargetMixin</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="nd">@classproperty</span>
<span class="k">def</span> <span class="nf">target_id</span><span class="p">(</span><span class="n">cls</span><span class="p">):</span>
<span class="k">return</span> <span class="n">Column</span><span class="p">(</span><span class="s">'target_id'</span><span class="p">,</span> <span class="n">ForeignKey</span><span class="p">(</span><span class="s">'target.id'</span><span class="p">))</span>
<span class="nd">@classproperty</span>
<span class="k">def</span> <span class="nf">target</span><span class="p">(</span><span class="n">cls</span><span class="p">):</span>
<span class="k">return</span> <span class="n">relationship</span><span class="p">(</span><span class="s">"Target"</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">Foo</span><span class="p">(</span><span class="n">Base</span><span class="p">,</span> <span class="n">RefTargetMixin</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'foo'</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">Bar</span><span class="p">(</span><span class="n">Base</span><span class="p">,</span> <span class="n">RefTargetMixin</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'bar'</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">Target</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">'target'</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span></pre></div>
</div>
<p><a class="reference internal" href="../orm/mapping.html#sqlalchemy.orm.relationship" title="sqlalchemy.orm.relationship"><tt class="xref py py-func docutils literal"><span class="pre">relationship()</span></tt></a> definitions which require explicit
primaryjoin, order_by etc. expressions should use the string forms
for these arguments, so that they are evaluated as late as possible.
To reference the mixin class in these expressions, use the given <tt class="docutils literal"><span class="pre">cls</span></tt>
to get it’s name:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">RefTargetMixin</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="nd">@classproperty</span>
<span class="k">def</span> <span class="nf">target_id</span><span class="p">(</span><span class="n">cls</span><span class="p">):</span>
<span class="k">return</span> <span class="n">Column</span><span class="p">(</span><span class="s">'target_id'</span><span class="p">,</span> <span class="n">ForeignKey</span><span class="p">(</span><span class="s">'target.id'</span><span class="p">))</span>
<span class="nd">@classproperty</span>
<span class="k">def</span> <span class="nf">target</span><span class="p">(</span><span class="n">cls</span><span class="p">):</span>
<span class="k">return</span> <span class="n">relationship</span><span class="p">(</span><span class="s">"Target"</span><span class="p">,</span>
<span class="n">primaryjoin</span><span class="o">=</span><span class="s">"Target.id==</span><span class="si">%s</span><span class="s">.target_id"</span> <span class="o">%</span> <span class="n">cls</span><span class="o">.</span><span class="n">__name__</span>
<span class="p">)</span></pre></div>
</div>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">The usage of <a class="reference internal" href="../sqlalchemy/util.html#sqlalchemy.util.classproperty" title="sqlalchemy.util.classproperty"><tt class="xref py py-func docutils literal"><span class="pre">classproperty()</span></tt></a> with mixin
relationships is a new feature as of SQLAlchemy 0.6.2.</p>
</div>
</div>
<div class="section" id="mixing-in-deferred-column-property-etc">
<h3>Mixing in deferred(), column_property(), etc.<a class="headerlink" href="#mixing-in-deferred-column-property-etc" title="Permalink to this headline">¶</a></h3>
<p>Like <a class="reference internal" href="../orm/mapping.html#sqlalchemy.orm.relationship" title="sqlalchemy.orm.relationship"><tt class="xref py py-func docutils literal"><span class="pre">relationship()</span></tt></a>, all
<a class="reference internal" href="../orm/mapping.html#sqlalchemy.orm.interfaces.MapperProperty" title="sqlalchemy.orm.interfaces.MapperProperty"><tt class="xref py py-class docutils literal"><span class="pre">MapperProperty</span></tt></a> subclasses such as
<a class="reference internal" href="../orm/mapping.html#sqlalchemy.orm.deferred" title="sqlalchemy.orm.deferred"><tt class="xref py py-func docutils literal"><span class="pre">deferred()</span></tt></a>, <a class="reference internal" href="../orm/mapping.html#sqlalchemy.orm.column_property" title="sqlalchemy.orm.column_property"><tt class="xref py py-func docutils literal"><span class="pre">column_property()</span></tt></a>,
etc. ultimately involve references to columns, and therefore have the
<a class="reference internal" href="../sqlalchemy/util.html#sqlalchemy.util.classproperty" title="sqlalchemy.util.classproperty"><tt class="xref py py-func docutils literal"><span class="pre">classproperty()</span></tt></a> requirement so that no reliance on
copying is needed:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">SomethingMixin</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="nd">@classproperty</span>
<span class="k">def</span> <span class="nf">dprop</span><span class="p">(</span><span class="n">cls</span><span class="p">):</span>
<span class="k">return</span> <span class="n">deferred</span><span class="p">(</span><span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">))</span>
<span class="k">class</span> <span class="nc">Something</span><span class="p">(</span><span class="n">Base</span><span class="p">,</span> <span class="n">SomethingMixin</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s">"something"</span></pre></div>
</div>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">The usage of <a class="reference internal" href="../sqlalchemy/util.html#sqlalchemy.util.classproperty" title="sqlalchemy.util.classproperty"><tt class="xref py py-func docutils literal"><span class="pre">classproperty()</span></tt></a> with mixin
mapper properties is a new feature as of SQLAlchemy 0.6.2.</p>
</div>
</div>
<div class="section" id="controlling-table-inheritance-with-mixins">
<h3>Controlling table inheritance with mixins<a class="headerlink" href="#controlling-table-inheritance-with-mixins" title="Permalink to this headline">¶</a></h3>
<p>The <tt class="docutils literal"><span class="pre">__tablename__</span></tt> attribute in conjunction with the hierarchy of
the classes involved controls what type of table inheritance, if any,
is configured by the declarative extension.</p>
<p>If the <tt class="docutils literal"><span class="pre">__tablename__</span></tt> is computed by a mixin, you may need to
control which classes get the computed attribute in order to get the
type of table inheritance you require.</p>
<p>For example, if you had a mixin that computes <tt class="docutils literal"><span class="pre">__tablename__</span></tt> but
where you wanted to use that mixin in a single table inheritance
hierarchy, you can explicitly specify <tt class="docutils literal"><span class="pre">__tablename__</span></tt> as <tt class="xref docutils literal"><span class="pre">None</span></tt> to
indicate that the class should not have a table mapped:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">sqlalchemy.util</span> <span class="kn">import</span> <span class="n">classproperty</span>
<span class="k">class</span> <span class="nc">Tablename</span><span class="p">:</span>
<span class="nd">@classproperty</span>
<span class="k">def</span> <span class="nf">__tablename__</span><span class="p">(</span><span class="n">cls</span><span class="p">):</span>
<span class="k">return</span> <span class="n">cls</span><span class="o">.</span><span class="n">__name__</span><span class="o">.</span><span class="n">lower</span><span class="p">()</span>
<span class="k">class</span> <span class="nc">Person</span><span class="p">(</span><span class="n">Base</span><span class="p">,</span><span class="n">Tablename</span><span class="p">):</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="n">discriminator</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="s">'type'</span><span class="p">,</span> <span class="n">String</span><span class="p">(</span><span class="mi">50</span><span class="p">))</span>
<span class="n">__mapper_args__</span> <span class="o">=</span> <span class="p">{</span><span class="s">'polymorphic_on'</span><span class="p">:</span> <span class="n">discriminator</span><span class="p">}</span>
<span class="k">class</span> <span class="nc">Engineer</span><span class="p">(</span><span class="n">Person</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="bp">None</span>
<span class="n">__mapper_args__</span> <span class="o">=</span> <span class="p">{</span><span class="s">'polymorphic_identity'</span><span class="p">:</span> <span class="s">'engineer'</span><span class="p">}</span>
<span class="n">primary_language</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">50</span><span class="p">))</span></pre></div>
</div>
<p>Alternatively, you can make the mixin intelligent enough to only
return a <tt class="docutils literal"><span class="pre">__tablename__</span></tt> in the event that no table is already
mapped in the inheritance hierarchy. To help with this, a
<a class="reference internal" href="#sqlalchemy.ext.declarative.has_inherited_table" title="sqlalchemy.ext.declarative.has_inherited_table"><tt class="xref py py-func docutils literal"><span class="pre">has_inherited_table()</span></tt></a> helper
function is provided that returns <tt class="xref docutils literal"><span class="pre">True</span></tt> if a parent class already
has a mapped table.</p>
<p>As an example, here’s a mixin that will only allow single table
inheritance:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">sqlalchemy.util</span> <span class="kn">import</span> <span class="n">classproperty</span>
<span class="kn">from</span> <span class="nn">sqlalchemy.ext.declarative</span> <span class="kn">import</span> <span class="n">has_inherited_table</span>
<span class="k">class</span> <span class="nc">Tablename</span><span class="p">:</span>
<span class="nd">@classproperty</span>
<span class="k">def</span> <span class="nf">__tablename__</span><span class="p">(</span><span class="n">cls</span><span class="p">):</span>
<span class="k">if</span> <span class="n">has_inherited_table</span><span class="p">(</span><span class="n">cls</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">None</span>
<span class="k">return</span> <span class="n">cls</span><span class="o">.</span><span class="n">__name__</span><span class="o">.</span><span class="n">lower</span><span class="p">()</span>
<span class="k">class</span> <span class="nc">Person</span><span class="p">(</span><span class="n">Base</span><span class="p">,</span><span class="n">Tablename</span><span class="p">):</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="n">discriminator</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="s">'type'</span><span class="p">,</span> <span class="n">String</span><span class="p">(</span><span class="mi">50</span><span class="p">))</span>
<span class="n">__mapper_args__</span> <span class="o">=</span> <span class="p">{</span><span class="s">'polymorphic_on'</span><span class="p">:</span> <span class="n">discriminator</span><span class="p">}</span>
<span class="k">class</span> <span class="nc">Engineer</span><span class="p">(</span><span class="n">Person</span><span class="p">):</span>
<span class="n">primary_language</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">50</span><span class="p">))</span>
<span class="n">__mapper_args__</span> <span class="o">=</span> <span class="p">{</span><span class="s">'polymorphic_identity'</span><span class="p">:</span> <span class="s">'engineer'</span><span class="p">}</span></pre></div>
</div>
<p>If you want to use a similar pattern with a mix of single and joined
table inheritance, you would need a slightly different mixin and use
it on any joined table child classes in addition to their parent
classes:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">sqlalchemy.util</span> <span class="kn">import</span> <span class="n">classproperty</span>
<span class="kn">from</span> <span class="nn">sqlalchemy.ext.declarative</span> <span class="kn">import</span> <span class="n">has_inherited_table</span>
<span class="k">class</span> <span class="nc">Tablename</span><span class="p">:</span>
<span class="nd">@classproperty</span>
<span class="k">def</span> <span class="nf">__tablename__</span><span class="p">(</span><span class="n">cls</span><span class="p">):</span>
<span class="k">if</span> <span class="p">(</span><span class="n">has_inherited_table</span><span class="p">(</span><span class="n">cls</span><span class="p">)</span> <span class="ow">and</span>
<span class="n">Tablename</span> <span class="ow">not</span> <span class="ow">in</span> <span class="n">cls</span><span class="o">.</span><span class="n">__bases__</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">None</span>
<span class="k">return</span> <span class="n">cls</span><span class="o">.</span><span class="n">__name__</span><span class="o">.</span><span class="n">lower</span><span class="p">()</span>
<span class="k">class</span> <span class="nc">Person</span><span class="p">(</span><span class="n">Base</span><span class="p">,</span><span class="n">Tablename</span><span class="p">):</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="n">discriminator</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="s">'type'</span><span class="p">,</span> <span class="n">String</span><span class="p">(</span><span class="mi">50</span><span class="p">))</span>
<span class="n">__mapper_args__</span> <span class="o">=</span> <span class="p">{</span><span class="s">'polymorphic_on'</span><span class="p">:</span> <span class="n">discriminator</span><span class="p">}</span>
<span class="c"># This is single table inheritance</span>
<span class="k">class</span> <span class="nc">Engineer</span><span class="p">(</span><span class="n">Person</span><span class="p">):</span>
<span class="n">primary_language</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">50</span><span class="p">))</span>
<span class="n">__mapper_args__</span> <span class="o">=</span> <span class="p">{</span><span class="s">'polymorphic_identity'</span><span class="p">:</span> <span class="s">'engineer'</span><span class="p">}</span>
<span class="c"># This is joined table inheritance</span>
<span class="k">class</span> <span class="nc">Manager</span><span class="p">(</span><span class="n">Person</span><span class="p">,</span><span class="n">Tablename</span><span class="p">):</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">ForeignKey</span><span class="p">(</span><span class="s">'person.id'</span><span class="p">),</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="n">preferred_recreation</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">50</span><span class="p">))</span>
<span class="n">__mapper_args__</span> <span class="o">=</span> <span class="p">{</span><span class="s">'polymorphic_identity'</span><span class="p">:</span> <span class="s">'engineer'</span><span class="p">}</span></pre></div>
</div>
</div>
<div class="section" id="combining-table-mapper-arguments-from-multiple-mixins">
<h3>Combining Table/Mapper Arguments from Multiple Mixins<a class="headerlink" href="#combining-table-mapper-arguments-from-multiple-mixins" title="Permalink to this headline">¶</a></h3>
<p>In the case of <tt class="docutils literal"><span class="pre">__table_args__</span></tt> or <tt class="docutils literal"><span class="pre">__mapper_args__</span></tt>, you may want
to combine some parameters from several mixins with those you wish to
define on the class iteself. The
<a class="reference internal" href="../sqlalchemy/util.html#sqlalchemy.util.classproperty" title="sqlalchemy.util.classproperty"><tt class="xref py py-func docutils literal"><span class="pre">classproperty()</span></tt></a> decorator can be used here
to create user-defined collation routines that pull from multiple
collections:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">sqlalchemy.util</span> <span class="kn">import</span> <span class="n">classproperty</span>
<span class="k">class</span> <span class="nc">MySQLSettings</span><span class="p">:</span>
<span class="n">__table_args__</span> <span class="o">=</span> <span class="p">{</span><span class="s">'mysql_engine'</span><span class="p">:</span><span class="s">'InnoDB'</span><span class="p">}</span>
<span class="k">class</span> <span class="nc">MyOtherMixin</span><span class="p">:</span>
<span class="n">__table_args__</span> <span class="o">=</span> <span class="p">{</span><span class="s">'info'</span><span class="p">:</span><span class="s">'foo'</span><span class="p">}</span>
<span class="k">class</span> <span class="nc">MyModel</span><span class="p">(</span><span class="n">Base</span><span class="p">,</span><span class="n">MySQLSettings</span><span class="p">,</span><span class="n">MyOtherMixin</span><span class="p">):</span>
<span class="n">__tablename__</span><span class="o">=</span><span class="s">'my_model'</span>
<span class="nd">@classproperty</span>
<span class="k">def</span> <span class="nf">__table_args__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="n">args</span> <span class="o">=</span> <span class="nb">dict</span><span class="p">()</span>
<span class="n">args</span><span class="o">.</span><span class="n">update</span><span class="p">(</span><span class="n">MySQLSettings</span><span class="o">.</span><span class="n">__table_args__</span><span class="p">)</span>
<span class="n">args</span><span class="o">.</span><span class="n">update</span><span class="p">(</span><span class="n">MyOtherMixin</span><span class="o">.</span><span class="n">__table_args__</span><span class="p">)</span>
<span class="k">return</span> <span class="n">args</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span></pre></div>
</div>
</div>
</div>
<div class="section" id="class-constructor">
<h2>Class Constructor<a class="headerlink" href="#class-constructor" title="Permalink to this headline">¶</a></h2>
<p>As a convenience feature, the <a class="reference internal" href="#sqlalchemy.ext.declarative.declarative_base" title="sqlalchemy.ext.declarative.declarative_base"><tt class="xref py py-func docutils literal"><span class="pre">declarative_base()</span></tt></a> sets a default
constructor on classes which takes keyword arguments, and assigns them
to the named attributes:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">e</span> <span class="o">=</span> <span class="n">Engineer</span><span class="p">(</span><span class="n">primary_language</span><span class="o">=</span><span class="s">'python'</span><span class="p">)</span></pre></div>
</div>
</div>
<div class="section" id="sessions">
<h2>Sessions<a class="headerlink" href="#sessions" title="Permalink to this headline">¶</a></h2>
<p>Note that <tt class="docutils literal"><span class="pre">declarative</span></tt> does nothing special with sessions, and is
only intended as an easier way to configure mappers and
<a class="reference internal" href="../sqlalchemy/schema.html#sqlalchemy.schema.Table" title="sqlalchemy.schema.Table"><tt class="xref py py-class docutils literal"><span class="pre">Table</span></tt></a> objects. A typical application
setup using <a class="reference internal" href="../orm/sessions.html#sqlalchemy.orm.scoped_session" title="sqlalchemy.orm.scoped_session"><tt class="xref py py-func docutils literal"><span class="pre">scoped_session()</span></tt></a> might look like:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">engine</span> <span class="o">=</span> <span class="n">create_engine</span><span class="p">(</span><span class="s">'postgresql://scott:tiger@localhost/test'</span><span class="p">)</span>
<span class="n">Session</span> <span class="o">=</span> <span class="n">scoped_session</span><span class="p">(</span><span class="n">sessionmaker</span><span class="p">(</span><span class="n">autocommit</span><span class="o">=</span><span class="bp">False</span><span class="p">,</span>
<span class="n">autoflush</span><span class="o">=</span><span class="bp">False</span><span class="p">,</span>
<span class="n">bind</span><span class="o">=</span><span class="n">engine</span><span class="p">))</span>
<span class="n">Base</span> <span class="o">=</span> <span class="n">declarative_base</span><span class="p">()</span></pre></div>
</div>
<p>Mapped instances then make usage of
<a class="reference internal" href="../orm/sessions.html#sqlalchemy.orm.session.Session" title="sqlalchemy.orm.session.Session"><tt class="xref py py-class docutils literal"><span class="pre">Session</span></tt></a> in the usual way.</p>
</div>
<div class="section" id="api-reference">
<h2>API Reference<a class="headerlink" href="#api-reference" title="Permalink to this headline">¶</a></h2>
<dl class="function">
<dt id="sqlalchemy.ext.declarative.declarative_base">
<tt class="descclassname">sqlalchemy.ext.declarative.</tt><tt class="descname">declarative_base</tt><big>(</big><em>bind=None</em>, <em>metadata=None</em>, <em>mapper=None</em>, <em>cls=<type 'object'></em>, <em>name='Base'</em>, <em>constructor=<function __init__ at 0x498db70></em>, <em>metaclass=<class 'sqlalchemy.ext.declarative.DeclarativeMeta'></em><big>)</big><a class="headerlink" href="#sqlalchemy.ext.declarative.declarative_base" title="Permalink to this definition">¶</a></dt>
<dd><p>Construct a base class for declarative class definitions.</p>
<p>The new base class will be given a metaclass that produces
appropriate <a class="reference internal" href="../sqlalchemy/schema.html#sqlalchemy.schema.Table" title="sqlalchemy.schema.Table"><tt class="xref py py-class docutils literal"><span class="pre">Table</span></tt></a> objects and makes
the appropriate <a class="reference internal" href="../orm/mapping.html#sqlalchemy.orm.mapper" title="sqlalchemy.orm.mapper"><tt class="xref py py-func docutils literal"><span class="pre">mapper()</span></tt></a> calls based on the
information provided declaratively in the class and any subclasses
of the class.</p>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Parameters:</th><td class="field-body"><ul class="first last simple">
<li><strong>bind</strong> – An optional
<a class="reference internal" href="../sqlalchemy/connections.html#sqlalchemy.engine.base.Connectable" title="sqlalchemy.engine.base.Connectable"><tt class="xref py py-class docutils literal"><span class="pre">Connectable</span></tt></a>, will be assigned
the <tt class="docutils literal"><span class="pre">bind</span></tt> attribute on the <tt class="xref py py-class docutils literal"><span class="pre">MetaData</span></tt>
instance.</li>
<li><strong>metadata</strong> – An optional <tt class="xref py py-class docutils literal"><span class="pre">MetaData</span></tt> instance. All
<a class="reference internal" href="../sqlalchemy/schema.html#sqlalchemy.schema.Table" title="sqlalchemy.schema.Table"><tt class="xref py py-class docutils literal"><span class="pre">Table</span></tt></a> objects implicitly declared by
subclasses of the base will share this MetaData. A MetaData instance
will be created if none is provided. The
<tt class="xref py py-class docutils literal"><span class="pre">MetaData</span></tt> instance will be available via the
<cite>metadata</cite> attribute of the generated declarative base class.</li>
<li><strong>mapper</strong> – An optional callable, defaults to <a class="reference internal" href="../orm/mapping.html#sqlalchemy.orm.mapper" title="sqlalchemy.orm.mapper"><tt class="xref py py-func docutils literal"><span class="pre">mapper()</span></tt></a>. Will
be used to map subclasses to their Tables.</li>
<li><strong>cls</strong> – Defaults to <tt class="xref py py-class docutils literal"><span class="pre">object</span></tt>. A type to use as the base for the generated
declarative base class. May be a class or tuple of classes.</li>
<li><strong>name</strong> – Defaults to <tt class="docutils literal"><span class="pre">Base</span></tt>. The display name for the generated
class. Customizing this is not required, but can improve clarity in
tracebacks and debugging.</li>
<li><strong>constructor</strong> – Defaults to
<a class="reference internal" href="#sqlalchemy.ext.declarative._declarative_constructor" title="sqlalchemy.ext.declarative._declarative_constructor"><tt class="xref py py-func docutils literal"><span class="pre">_declarative_constructor()</span></tt></a>, an
__init__ implementation that assigns **kwargs for declared
fields and relationships to an instance. If <tt class="xref docutils literal"><span class="pre">None</span></tt> is supplied,
no __init__ will be provided and construction will fall back to
cls.__init__ by way of the normal Python semantics.</li>
<li><strong>metaclass</strong> – Defaults to <tt class="xref py py-class docutils literal"><span class="pre">DeclarativeMeta</span></tt>. A metaclass or __metaclass__
compatible callable to use as the meta type of the generated
declarative base class.</li>
</ul>
</td>
</tr>
</tbody>
</table>
</dd></dl>
<dl class="function">
<dt id="sqlalchemy.ext.declarative._declarative_constructor">
<tt class="descclassname">sqlalchemy.ext.declarative.</tt><tt class="descname">_declarative_constructor</tt><big>(</big><em>self</em>, <em>**kwargs</em><big>)</big><a class="headerlink" href="#sqlalchemy.ext.declarative._declarative_constructor" title="Permalink to this definition">¶</a></dt>
<dd><p>A simple constructor that allows initialization from kwargs.</p>
<p>Sets attributes on the constructed instance using the names and
values in <tt class="docutils literal"><span class="pre">kwargs</span></tt>.</p>
<p>Only keys that are present as
attributes of the instance’s class are allowed. These could be,
for example, any mapped columns or relationships.</p>
</dd></dl>
<dl class="function">
<dt id="sqlalchemy.ext.declarative.has_inherited_table">
<tt class="descclassname">sqlalchemy.ext.declarative.</tt><tt class="descname">has_inherited_table</tt><big>(</big><em>cls</em><big>)</big><a class="headerlink" href="#sqlalchemy.ext.declarative.has_inherited_table" title="Permalink to this definition">¶</a></dt>
<dd><p>Given a class, return True if any of the classes it inherits from has a
mapped table, otherwise return False.</p>
</dd></dl>
<dl class="function">
<dt id="sqlalchemy.ext.declarative.synonym_for">
<tt class="descclassname">sqlalchemy.ext.declarative.</tt><tt class="descname">synonym_for</tt><big>(</big><em>name</em>, <em>map_column=False</em><big>)</big><a class="headerlink" href="#sqlalchemy.ext.declarative.synonym_for" title="Permalink to this definition">¶</a></dt>
<dd><p>Decorator, make a Python @property a query synonym for a column.</p>
<p>A decorator version of <a class="reference internal" href="../orm/mapping.html#sqlalchemy.orm.synonym" title="sqlalchemy.orm.synonym"><tt class="xref py py-func docutils literal"><span class="pre">synonym()</span></tt></a>. The function being
decorated is the ‘descriptor’, otherwise passes its arguments through to
synonym():</p>
<div class="highlight-python"><div class="highlight"><pre><span class="nd">@synonym_for</span><span class="p">(</span><span class="s">'col'</span><span class="p">)</span>
<span class="nd">@property</span>
<span class="k">def</span> <span class="nf">prop</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="s">'special sauce'</span></pre></div>
</div>
<p>The regular <tt class="docutils literal"><span class="pre">synonym()</span></tt> is also usable directly in a declarative setting
and may be convenient for read/write properties:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">prop</span> <span class="o">=</span> <span class="n">synonym</span><span class="p">(</span><span class="s">'col'</span><span class="p">,</span> <span class="n">descriptor</span><span class="o">=</span><span class="nb">property</span><span class="p">(</span><span class="n">_read_prop</span><span class="p">,</span> <span class="n">_write_prop</span><span class="p">))</span></pre></div>
</div>
</dd></dl>
<dl class="function">
<dt id="sqlalchemy.ext.declarative.comparable_using">
<tt class="descclassname">sqlalchemy.ext.declarative.</tt><tt class="descname">comparable_using</tt><big>(</big><em>comparator_factory</em><big>)</big><a class="headerlink" href="#sqlalchemy.ext.declarative.comparable_using" title="Permalink to this definition">¶</a></dt>
<dd><p>Decorator, allow a Python @property to be used in query criteria.</p>
<p>This is a decorator front end to
<a class="reference internal" href="../orm/mapping.html#sqlalchemy.orm.comparable_property" title="sqlalchemy.orm.comparable_property"><tt class="xref py py-func docutils literal"><span class="pre">comparable_property()</span></tt></a> that passes
through the comparator_factory and the function being decorated:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="nd">@comparable_using</span><span class="p">(</span><span class="n">MyComparatorType</span><span class="p">)</span>
<span class="nd">@property</span>
<span class="k">def</span> <span class="nf">prop</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="s">'special sauce'</span></pre></div>
</div>
<p>The regular <tt class="docutils literal"><span class="pre">comparable_property()</span></tt> is also usable directly in a
declarative setting and may be convenient for read/write properties:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">prop</span> <span class="o">=</span> <span class="n">comparable_property</span><span class="p">(</span><span class="n">MyComparatorType</span><span class="p">)</span></pre></div>
</div>
</dd></dl>
<dl class="function">
<dt id="sqlalchemy.ext.declarative.instrument_declarative">
<tt class="descclassname">sqlalchemy.ext.declarative.</tt><tt class="descname">instrument_declarative</tt><big>(</big><em>cls</em>, <em>registry</em>, <em>metadata</em><big>)</big><a class="headerlink" href="#sqlalchemy.ext.declarative.instrument_declarative" title="Permalink to this definition">¶</a></dt>
<dd><p>Given a class, configure the class declaratively,
using the given registry, which can be any dictionary, and
MetaData object.</p>
</dd></dl>
</div>
</div>
</div>
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