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<h1>Source code for examples.generic_associations.generic_fk</h1><div class="highlight"><pre>
<span class="sd">&quot;&quot;&quot;generic_fk.py</span>

<span class="sd">Illustrates a so-called &quot;generic foreign key&quot;, in a similar fashion</span>
<span class="sd">to that of popular frameworks such as Django, ROR, etc.  This</span>
<span class="sd">approach bypasses standard referential integrity</span>
<span class="sd">practices, in that the &quot;foreign key&quot; column is not actually</span>
<span class="sd">constrained to refer to any particular table; instead,</span>
<span class="sd">in-application logic is used to determine which table is referenced.</span>

<span class="sd">This approach is not in line with SQLAlchemy&#39;s usual style, as foregoing</span>
<span class="sd">foreign key integrity means that the tables can easily contain invalid</span>
<span class="sd">references and also have no ability to use in-database cascade functionality.</span>

<span class="sd">However, due to the popularity of these systems, as well as that it uses</span>
<span class="sd">the fewest number of tables (which doesn&#39;t really offer any &quot;advantage&quot;,</span>
<span class="sd">though seems to be comforting to many) this recipe remains in</span>
<span class="sd">high demand, so in the interests of having an easy StackOverflow answer</span>
<span class="sd">queued up, here it is.   The author recommends &quot;table_per_related&quot;</span>
<span class="sd">or &quot;table_per_association&quot; instead of this approach.</span>

<span class="sd">.. versionadded:: 0.8.3</span>

<span class="sd">&quot;&quot;&quot;</span>
<span class="kn">from</span> <span class="nn">sqlalchemy.ext.declarative</span> <span class="kn">import</span> <span class="n">as_declarative</span><span class="p">,</span> <span class="n">declared_attr</span>
<span class="kn">from</span> <span class="nn">sqlalchemy</span> <span class="kn">import</span> <span class="n">create_engine</span><span class="p">,</span> <span class="n">Integer</span><span class="p">,</span> <span class="n">Column</span><span class="p">,</span> \
                    <span class="n">String</span><span class="p">,</span> <span class="n">and_</span>
<span class="kn">from</span> <span class="nn">sqlalchemy.orm</span> <span class="kn">import</span> <span class="n">Session</span><span class="p">,</span> <span class="n">relationship</span><span class="p">,</span> <span class="n">foreign</span><span class="p">,</span> <span class="n">remote</span><span class="p">,</span> <span class="n">backref</span>
<span class="kn">from</span> <span class="nn">sqlalchemy</span> <span class="kn">import</span> <span class="n">event</span>


<span class="nd">@as_declarative</span><span class="p">()</span>
<span class="k">class</span> <span class="nc">Base</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
    <span class="sd">&quot;&quot;&quot;Base class which provides automated table name</span>
<span class="sd">    and surrogate primary key column.</span>

<span class="sd">    &quot;&quot;&quot;</span>
    <span class="nd">@declared_attr</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="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">Address</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
    <span class="sd">&quot;&quot;&quot;The Address class.</span>

<span class="sd">    This represents all address records in a</span>
<span class="sd">    single table.</span>

<span class="sd">    &quot;&quot;&quot;</span>
    <span class="n">street</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="n">city</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="nb">zip</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="n">discriminator</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="sd">&quot;&quot;&quot;Refers to the type of parent.&quot;&quot;&quot;</span>

    <span class="n">parent_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="sd">&quot;&quot;&quot;Refers to the primary key of the parent.</span>

<span class="sd">    This could refer to any table.</span>
<span class="sd">    &quot;&quot;&quot;</span>

    <span class="nd">@property</span>
    <span class="k">def</span> <span class="nf">parent</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="sd">&quot;&quot;&quot;Provides in-Python access to the &quot;parent&quot; by choosing</span>
<span class="sd">        the appropriate relationship.</span>

<span class="sd">        &quot;&quot;&quot;</span>
        <span class="k">return</span> <span class="nb">getattr</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="s">&quot;parent_</span><span class="si">%s</span><span class="s">&quot;</span> <span class="o">%</span> <span class="bp">self</span><span class="o">.</span><span class="n">discriminator</span><span class="p">)</span>

    <span class="k">def</span> <span class="nf">__repr__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="k">return</span> <span class="s">&quot;</span><span class="si">%s</span><span class="s">(street=</span><span class="si">%r</span><span class="s">, city=</span><span class="si">%r</span><span class="s">, zip=</span><span class="si">%r</span><span class="s">)&quot;</span> <span class="o">%</span> \
            <span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">__class__</span><span class="o">.</span><span class="n">__name__</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">street</span><span class="p">,</span>
            <span class="bp">self</span><span class="o">.</span><span class="n">city</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">zip</span><span class="p">)</span>

<span class="k">class</span> <span class="nc">HasAddresses</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
    <span class="sd">&quot;&quot;&quot;HasAddresses mixin, creates a relationship to</span>
<span class="sd">    the address_association table for each parent.</span>

<span class="sd">    &quot;&quot;&quot;</span>

<span class="nd">@event.listens_for</span><span class="p">(</span><span class="n">HasAddresses</span><span class="p">,</span> <span class="s">&quot;mapper_configured&quot;</span><span class="p">,</span> <span class="n">propagate</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">setup_listener</span><span class="p">(</span><span class="n">mapper</span><span class="p">,</span> <span class="n">class_</span><span class="p">):</span>
    <span class="n">name</span> <span class="o">=</span> <span class="n">class_</span><span class="o">.</span><span class="n">__name__</span>
    <span class="n">discriminator</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="n">class_</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">and_</span><span class="p">(</span>
                                        <span class="n">class_</span><span class="o">.</span><span class="n">id</span> <span class="o">==</span> <span class="n">foreign</span><span class="p">(</span><span class="n">remote</span><span class="p">(</span><span class="n">Address</span><span class="o">.</span><span class="n">parent_id</span><span class="p">)),</span>
                                        <span class="n">Address</span><span class="o">.</span><span class="n">discriminator</span> <span class="o">==</span> <span class="n">discriminator</span>
                                    <span class="p">),</span>
                        <span class="n">backref</span><span class="o">=</span><span class="n">backref</span><span class="p">(</span>
                                <span class="s">&quot;parent_</span><span class="si">%s</span><span class="s">&quot;</span> <span class="o">%</span> <span class="n">discriminator</span><span class="p">,</span>
                                <span class="n">primaryjoin</span><span class="o">=</span><span class="n">remote</span><span class="p">(</span><span class="n">class_</span><span class="o">.</span><span class="n">id</span><span class="p">)</span> <span class="o">==</span> <span class="n">foreign</span><span class="p">(</span><span class="n">Address</span><span class="o">.</span><span class="n">parent_id</span><span class="p">)</span>
                                <span class="p">)</span>
                        <span class="p">)</span>
    <span class="nd">@event.listens_for</span><span class="p">(</span><span class="n">class_</span><span class="o">.</span><span class="n">addresses</span><span class="p">,</span> <span class="s">&quot;append&quot;</span><span class="p">)</span>
    <span class="k">def</span> <span class="nf">append_address</span><span class="p">(</span><span class="n">target</span><span class="p">,</span> <span class="n">value</span><span class="p">,</span> <span class="n">initiator</span><span class="p">):</span>
        <span class="n">value</span><span class="o">.</span><span class="n">discriminator</span> <span class="o">=</span> <span class="n">discriminator</span>

<span class="k">class</span> <span class="nc">Customer</span><span class="p">(</span><span class="n">HasAddresses</span><span class="p">,</span> <span class="n">Base</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="k">class</span> <span class="nc">Supplier</span><span class="p">(</span><span class="n">HasAddresses</span><span class="p">,</span> <span class="n">Base</span><span class="p">):</span>
    <span class="n">company_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="n">engine</span> <span class="o">=</span> <span class="n">create_engine</span><span class="p">(</span><span class="s">&#39;sqlite://&#39;</span><span class="p">,</span> <span class="n">echo</span><span class="o">=</span><span class="bp">True</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>

<span class="n">session</span> <span class="o">=</span> <span class="n">Session</span><span class="p">(</span><span class="n">engine</span><span class="p">)</span>

<span class="n">session</span><span class="o">.</span><span class="n">add_all</span><span class="p">([</span>
    <span class="n">Customer</span><span class="p">(</span>
        <span class="n">name</span><span class="o">=</span><span class="s">&#39;customer 1&#39;</span><span class="p">,</span>
        <span class="n">addresses</span><span class="o">=</span><span class="p">[</span>
            <span class="n">Address</span><span class="p">(</span>
                    <span class="n">street</span><span class="o">=</span><span class="s">&#39;123 anywhere street&#39;</span><span class="p">,</span>
                    <span class="n">city</span><span class="o">=</span><span class="s">&quot;New York&quot;</span><span class="p">,</span>
                    <span class="nb">zip</span><span class="o">=</span><span class="s">&quot;10110&quot;</span><span class="p">),</span>
            <span class="n">Address</span><span class="p">(</span>
                    <span class="n">street</span><span class="o">=</span><span class="s">&#39;40 main street&#39;</span><span class="p">,</span>
                    <span class="n">city</span><span class="o">=</span><span class="s">&quot;San Francisco&quot;</span><span class="p">,</span>
                    <span class="nb">zip</span><span class="o">=</span><span class="s">&quot;95732&quot;</span><span class="p">)</span>
        <span class="p">]</span>
    <span class="p">),</span>
    <span class="n">Supplier</span><span class="p">(</span>
        <span class="n">company_name</span><span class="o">=</span><span class="s">&quot;Ace Hammers&quot;</span><span class="p">,</span>
        <span class="n">addresses</span><span class="o">=</span><span class="p">[</span>
            <span class="n">Address</span><span class="p">(</span>
                    <span class="n">street</span><span class="o">=</span><span class="s">&#39;2569 west elm&#39;</span><span class="p">,</span>
                    <span class="n">city</span><span class="o">=</span><span class="s">&quot;Detroit&quot;</span><span class="p">,</span>
                    <span class="nb">zip</span><span class="o">=</span><span class="s">&quot;56785&quot;</span><span class="p">)</span>
        <span class="p">]</span>
    <span class="p">),</span>
<span class="p">])</span>

<span class="n">session</span><span class="o">.</span><span class="n">commit</span><span class="p">()</span>

<span class="k">for</span> <span class="n">customer</span> <span class="ow">in</span> <span class="n">session</span><span class="o">.</span><span class="n">query</span><span class="p">(</span><span class="n">Customer</span><span class="p">):</span>
    <span class="k">for</span> <span class="n">address</span> <span class="ow">in</span> <span class="n">customer</span><span class="o">.</span><span class="n">addresses</span><span class="p">:</span>
        <span class="k">print</span><span class="p">(</span><span class="n">address</span><span class="p">)</span>
        <span class="k">print</span><span class="p">(</span><span class="n">address</span><span class="o">.</span><span class="n">parent</span><span class="p">)</span></pre></div>
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