===============
Model reference
===============

A model is the single, definitive source of data about your data. It contains
the essential fields and behaviors of the data you're storing. Generally, each
model maps to a single database table.

The basics:

    * Each model is a Python class that subclasses ``django.db.models.Model``.
    * Each attribute of the model represents a database field.
    * Model metadata (non-field information) goes in an inner class named
      ``Meta``.
    * Metadata used for Django's admin site goes into an inner class named
      ``Admin``.
    * With all of this, Django gives you an automatically-generated
      database-access API, which is explained in the `Database API reference`_.

A companion to this document is the `official repository of model examples`_.
(In the Django source distribution, these examples are in the
``tests/modeltests`` directory.)

.. _Database API reference: http://www.djangoproject.com/documentation/db_api/
.. _official repository of model examples: http://www.djangoproject.com/documentation/models/

Quick example
=============

This example model defines a ``Person``, which has a ``first_name`` and
``last_name``::

    from django.db import models

    class Person(models.Model):
        first_name = models.CharField(maxlength=30)
        last_name = models.CharField(maxlength=30)

``first_name`` and ``last_name`` are *fields* of the model. Each field is
specified as a class attribute, and each attribute maps to a database column.

The above ``Person`` model would create a database table like this::

    CREATE TABLE myapp_person (
        "id" serial NOT NULL PRIMARY KEY,
        "first_name" varchar(30) NOT NULL,
        "last_name" varchar(30) NOT NULL
    );

Some technical notes:

    * The name of the table, ``myapp_person``, is automatically derived from
      some model metadata but can be overridden. See _`Table names` below.
    * An ``id`` field is added automatically, but this behavior can be
      overriden. See `Automatic primary key fields`_ below.
    * The ``CREATE TABLE`` SQL in this example is formatted using PostgreSQL
      syntax, but it's worth noting Django uses SQL tailored to the database
      backend specified in your `settings file`_.

.. _settings file: http://www.djangoproject.com/documentation/settings/

Fields
======

The most important part of a model -- and the only required part of a model --
is the list of database fields it defines. Fields are specified by class
attributes.

Example::

    class Musician(models.Model):
        first_name = models.CharField(maxlength=50)
        last_name = models.CharField(maxlength=50)
        instrument = models.CharField(maxlength=100)

    class Album(models.Model):
        artist = models.ForeignKey(Musician)
        name = models.CharField(maxlength=100)
        release_date = models.DateField()
        num_stars = models.IntegerField()

Field name restrictions
-----------------------

Django places only two restrictions on model field names:

    1. A field name cannot be a Python reserved word, because that would result
       in a Python syntax error. For example::

           class Example(models.Model):
               pass = models.IntegerField() # 'pass' is a reserved word!

    2. A field name cannot contain more than one underscore in a row, due to
       the way Django's query lookup syntax works. For example::

           class Example(models.Model):
               foo__bar = models.IntegerField() 'foo__bar' has two underscores!

These limitations can be worked around, though, because your field name doesn't
necessarily have to match your database column name. See `db_column`_ below.

SQL reserved words, such as ``join``, ``where`` or ``select``, *are* allowed as
model field names, because Django escapes all database table names and column
names in every underlying SQL query. It uses the quoting syntax of your
particular database engine.

Field types
-----------

Each field in your model should be an instance of the appropriate ``Field``
class. Django uses the field class types to determine a few things:

    * The database column type (e.g. ``INTEGER``, ``VARCHAR``).
    * The widget to use in Django's admin interface, if you care to use it
      (e.g. ``<input type="text">``, ``<select>``).
    * The minimal validation requirements, used in Django's admin and in
      manipulators.

Here are all available field types:

``AutoField``
~~~~~~~~~~~~~

An ``IntegerField`` that automatically increments according to available IDs.
You usually won't need to use this directly; a primary key field will
automatically be added to your model if you don't specify otherwise. See
`Automatic primary key fields`_.

``BooleanField``
~~~~~~~~~~~~~~~~

A true/false field.

The admin represents this as a checkbox.

``CharField``
~~~~~~~~~~~~~

A string field, for small- to large-sized strings.

For large amounts of text, use ``TextField``.

The admin represents this as an ``<input type="text">`` (a single-line input).

``CharField`` has an extra required argument, ``maxlength``, the maximum length
(in characters) of the field. The maxlength is enforced at the database level
and in Django's validation.

``CommaSeparatedIntegerField``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

A field of integers separated by commas. As in ``CharField``, the ``maxlength``
argument is required.

``DateField``
~~~~~~~~~~~~~

A date field. Has a few extra optional arguments:

    ======================  ===================================================
    Argument                Description
    ======================  ===================================================
    ``auto_now``            Automatically set the field to now every time the
                            object is saved. Useful for "last-modified"
                            timestamps. Note that the current date is *always*
                            used; it's not just a default value that you can
                            override.

    ``auto_now_add``        Automatically set the field to now when the object
                            is first created. Useful for creation of
                            timestamps. Note that the current date is *always*
                            used; it's not just a default value that you can
                            override.
    ======================  ===================================================

The admin represents this as an ``<input type="text">`` with a JavaScript
calendar and a shortcut for "Today."

``DateTimeField``
~~~~~~~~~~~~~~~~~

A date and time field. Takes the same extra options as ``DateField``.

The admin represents this as two ``<input type="text">`` fields, with
JavaScript shortcuts.

``EmailField``
~~~~~~~~~~~~~~

A ``CharField`` that checks that the value is a valid e-mail address.
This doesn't accept ``maxlength``.

``FileField``
~~~~~~~~~~~~~

A file-upload field.

Has an extra required argument, ``upload_to``, a local filesystem path to
which files should be upload. This path may contain `strftime formatting`_,
which will be replaced by the date/time of the file upload (so that
uploaded files don't fill up the given directory).

The admin represents this as an ``<input type="file">`` (a file-upload widget).

Using a ``FileField`` or an ``ImageField`` (see below) in a model takes a few
steps:

    1. In your settings file, you'll need to define ``MEDIA_ROOT`` as the
       full path to a directory where you'd like Django to store uploaded
       files. (For performance, these files are not stored in the database.)
       Define ``MEDIA_URL`` as the base public URL of that directory. Make
       sure that this directory is writable by the Web server's user
       account.

    2. Add the ``FileField`` or ``ImageField`` to your model, making sure
       to define the ``upload_to`` option to tell Django to which
       subdirectory of ``MEDIA_ROOT`` it should upload files.

    3. All that will be stored in your database is a path to the file
       (relative to ``MEDIA_ROOT``). You'll must likely want to use the
       convenience ``get_<fieldname>_url`` function provided by Django. For
       example, if your ``ImageField`` is called ``mug_shot``, you can get
       the absolute URL to your image in a template with ``{{
       object.get_mug_shot_url }}``.

.. _`strftime formatting`: http://docs.python.org/lib/module-time.html#l2h-1941

``FilePathField``
~~~~~~~~~~~~~~~~~

A field whose choices are limited to the filenames in a certain directory
on the filesystem. Has three special arguments, of which the first is
required:

    ======================  ===================================================
    Argument                Description
    ======================  ===================================================
    ``path``                Required. The absolute filesystem path to a
                            directory from which this ``FilePathField`` should
                            get its choices. Example: ``"/home/images"``.

    ``match``               Optional. A regular expression, as a string, that
                            ``FilePathField`` will use to filter filenames.
                            Note that the regex will be applied to the
                            base filename, not the full path. Example:
                            ``"foo.*\.txt^"``, which will match a file called
                            ``foo23.txt`` but not ``bar.txt`` or ``foo23.gif``.

    ``recursive``           Optional. Either ``True`` or ``False``. Default is
                            ``False``. Specifies whether all subdirectories of
                            ``path`` should be included.
    ======================  ===================================================

Of course, these arguments can be used together.

The one potential gotcha is that ``match`` applies to the base filename,
not the full path. So, this example::

    FilePathField(path="/home/images", match="foo.*", recursive=True)

...will match ``/home/images/foo.gif`` but not ``/home/images/foo/bar.gif``
because the ``match`` applies to the base filename (``foo.gif`` and
``bar.gif``).

``FloatField``
~~~~~~~~~~~~~~

A floating-point number. Has two **required** arguments:

    ======================  ===================================================
    Argument                Description
    ======================  ===================================================
    ``max_digits``          The maximum number of digits allowed in the number.

    ``decimal_places``      The number of decimal places to store with the
                            number.
    ======================  ===================================================

For example, to store numbers up to 999 with a resolution of 2 decimal places,
you'd use::

    models.FloatField(..., max_digits=5, decimal_places=2)

And to store numbers up to approximately one billion with a resolution of 10
decimal places::

    models.FloatField(..., max_digits=19, decimal_places=10)

The admin represents this as an ``<input type="text">`` (a single-line input).

``ImageField``
~~~~~~~~~~~~~~

Like ``FileField``, but validates that the uploaded object is a valid
image. Has two extra optional arguments, ``height_field`` and
``width_field``, which, if set, will be auto-populated with the height and
width of the image each time a model instance is saved.

Requires the `Python Imaging Library`_.

.. _Python Imaging Library: http://www.pythonware.com/products/pil/

``IntegerField``
~~~~~~~~~~~~~~~~

An integer.

The admin represents this as an ``<input type="text">`` (a single-line input).

``IPAddressField``
~~~~~~~~~~~~~~~~~~

An IP address, in string format (i.e. "24.124.1.30").

The admin represents this as an ``<input type="text">`` (a single-line input).

``NullBooleanField``
~~~~~~~~~~~~~~~~~~~~

Like a ``BooleanField``, but allows ``NULL`` as one of the options.  Use this
instead of a ``BooleanField`` with ``null=True``.

The admin represents this as a ``<select>`` box with "Unknown", "Yes" and "No" choices.

``PhoneNumberField``
~~~~~~~~~~~~~~~~~~~~

A ``CharField`` that checks that the value is a valid U.S.A.-style phone
number (in the format ``XXX-XXX-XXXX``).

``PositiveIntegerField``
~~~~~~~~~~~~~~~~~~~~~~~~

Like an ``IntegerField``, but must be positive.

``PositiveSmallIntegerField``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Like a ``PositiveIntegerField``, but only allows values under a certain
(database-dependent) point.

``SlugField``
~~~~~~~~~~~~~

"Slug" is a newspaper term. A slug is a short label for something,
containing only letters, numbers, underscores or hyphens. They're generally
used in URLs.

In the Django development version, you can specify ``maxlength``. If
``maxlength`` is not specified, Django will use a default length of 50. In
previous Django versions, there's no way to override the length of 50.

Implies ``db_index=True``.

Accepts an extra option, ``prepopulate_from``, which is a list of fields
from which to auto-populate the slug, via JavaScript, in the object's admin
form::

    models.SlugField(prepopulate_from=("pre_name", "name"))

``prepopulate_from`` doesn't accept DateTimeFields.

The admin represents ``SlugField`` as an ``<input type="text">`` (a
single-line input).

``SmallIntegerField``
~~~~~~~~~~~~~~~~~~~~~

Like an ``IntegerField``, but only allows values under a certain
(database-dependent) point.

``TextField``
~~~~~~~~~~~~~

A large text field.

The admin represents this as a ``<textarea>`` (a multi-line input).

``TimeField``
~~~~~~~~~~~~~

A time. Accepts the same auto-population options as ``DateField`` and
``DateTimeField``.

The admin represents this as an ``<input type="text">`` with some
JavaScript shortcuts.

``URLField``
~~~~~~~~~~~~

A field for a URL. If the ``verify_exists`` option is ``True`` (default),
the URL given will be checked for existence (i.e., the URL actually loads
and doesn't give a 404 response).

The admin represents this as an ``<input type="text">`` (a single-line input).

``USStateField``
~~~~~~~~~~~~~~~~

A two-letter U.S. state abbreviation.

The admin represents this as an ``<input type="text">`` (a single-line input).

``XMLField``
~~~~~~~~~~~~

A ``TextField`` that checks that the value is valid XML that matches a
given schema. Takes one required argument, ``schema_path``, which is the
filesystem path to a RelaxNG_ schema against which to validate the field.

.. _RelaxNG: http://www.relaxng.org/

Field options
-------------

The following arguments are available to all field types. All are optional.

``null``
~~~~~~~~

If ``True``, Django will store empty values as ``NULL`` in the database.
Default is ``False``.

Note that empty string values will always get stored as empty strings, not
as ``NULL`` -- so use ``null=True`` for non-string fields such as integers,
booleans and dates.

Avoid using ``null`` on string-based fields such as ``CharField`` and
``TextField`` unless you have an excellent reason. If a string-based field
has ``null=True``, that means it has two possible values for "no data":
``NULL``, and the empty string. In most cases, it's redundant to have two
possible values for "no data;" Django convention is to use the empty
string, not ``NULL``.

``blank``
~~~~~~~~~

If ``True``, the field is allowed to be blank.

Note that this is different than ``null``. ``null`` is purely
database-related, whereas ``blank`` is validation-related. If a field has
``blank=True``, validation on Django's admin site will allow entry of an
empty value. If a field has ``blank=False``, the field will be required.

``choices``
~~~~~~~~~~~

An iterable (e.g., a list or tuple) of 2-tuples to use as choices for this
field.

If this is given, Django's admin will use a select box instead of the
standard text field and will limit choices to the choices given.

A choices list looks like this::

    YEAR_IN_SCHOOL_CHOICES = (
        ('FR', 'Freshman'),
        ('SO', 'Sophomore'),
        ('JR', 'Junior'),
        ('SR', 'Senior'),
        ('GR', 'Graduate'),
    )

The first element in each tuple is the actual value to be stored. The
second element is the human-readable name for the option.

The choices list can be defined either as part of your model class::

    class Foo(models.Model):
        GENDER_CHOICES = (
            ('M', 'Male'),
            ('F', 'Female'),
        )
        gender = models.CharField(maxlength=1, choices=GENDER_CHOICES)

or outside your model class altogether::

    GENDER_CHOICES = (
        ('M', 'Male'),
        ('F', 'Female'),
    )
    class Foo(models.Model):
        gender = models.CharField(maxlength=1, choices=GENDER_CHOICES)

Finally, note that choices can be any iterable object -- not necessarily a
list or tuple. This lets you construct choices dynamically. But if you find
yourself hacking ``choices`` to be dynamic, you're probably better off using
a proper database table with a ``ForeignKey``. ``choices`` is meant for static
data that doesn't change much, if ever.

``core``
~~~~~~~~

For objects that are edited inline to a related object.

In the Django admin, if all "core" fields in an inline-edited object are
cleared, the object will be deleted.

It is an error to have an inline-editable relation without at least one
``core=True`` field.

Please note that each field marked "core" is treated as a required field by the
Django admin site. Essentially, this means you should put ``core=True`` on all
required fields in your related object that is being edited inline.

``db_column``
~~~~~~~~~~~~~

The name of the database column to use for this field. If this isn't given,
Django will use the field's name.

If your database column name is an SQL reserved word, or contains
characters that aren't allowed in Python variable names -- notably, the
hyphen -- that's OK. Django quotes column and table names behind the
scenes.

``db_index``
~~~~~~~~~~~~

If ``True``, ``django-admin.py sqlindexes`` will output a ``CREATE INDEX``
statement for this field.

``default``
~~~~~~~~~~~

The default value for the field.

``editable``
~~~~~~~~~~~~

If ``False``, the field will not be editable in the admin. Default is  ``True``.

``help_text``
~~~~~~~~~~~~~

Extra "help" text to be displayed under the field on the object's admin
form. It's useful for documentation even if your object doesn't have an
admin form.

``primary_key``
~~~~~~~~~~~~~~~

If ``True``, this field is the primary key for the model.

If you don't specify ``primary_key=True`` for any fields in your model,
Django will automatically add this field::

    id = models.AutoField('ID', primary_key=True)

Thus, you don't need to set ``primary_key=True`` on any of your fields
unless you want to override the default primary-key behavior.

``primary_key=True`` implies ``blank=False``, ``null=False`` and
``unique=True``. Only one primary key is allowed on an object.

``radio_admin``
~~~~~~~~~~~~~~~

By default, Django's admin uses a select-box interface (<select>) for
fields that are ``ForeignKey`` or have ``choices`` set. If ``radio_admin``
is set to ``True``, Django will use a radio-button interface instead.

Don't use this for a field unless it's a ``ForeignKey`` or has ``choices``
set.

``unique``
~~~~~~~~~~

If ``True``, this field must be unique throughout the table.

This is enforced at the database level and at the Django admin-form level.

``unique_for_date``
~~~~~~~~~~~~~~~~~~~

Set this to the name of a ``DateField`` or ``DateTimeField`` to require
that this field be unique for the value of the date field.

For example, if you have a field ``title`` that has
``unique_for_date="pub_date"``, then Django wouldn't allow the entry of
two records with the same ``title`` and ``pub_date``.

This is enforced at the Django admin-form level but not at the database level.

``unique_for_month``
~~~~~~~~~~~~~~~~~~~~

Like ``unique_for_date``, but requires the field to be unique with respect
to the month.

``unique_for_year``
~~~~~~~~~~~~~~~~~~~

Like ``unique_for_date`` and ``unique_for_month``.

``validator_list``
~~~~~~~~~~~~~~~~~~

A list of extra validators to apply to the field. Each should be a callable
that takes the parameters ``field_data, all_data`` and raises
``django.core.validators.ValidationError`` for errors. (See the
`validator docs`_.)

Django comes with quite a few validators. They're in ``django.core.validators``.

.. _validator docs: http://www.djangoproject.com/documentation/forms/#validators

Verbose field names
-------------------

Each field type, except for ``ForeignKey``, ``ManyToManyField`` and
``OneToOneField``, takes an optional first positional argument -- a
verbose name. If the verbose name isn't given, Django will automatically create
it using the field's attribute name, converting underscores to spaces.

In this example, the verbose name is ``"Person's first name"``::

    first_name = models.CharField("Person's first name", maxlength=30)

In this example, the verbose name is ``"first name"``::

    first_name = models.CharField(maxlength=30)

``ForeignKey``, ``ManyToManyField`` and ``OneToOneField`` require the first
argument to be a model class, so use the ``verbose_name`` keyword argument::

    poll = models.ForeignKey(Poll, verbose_name="the related poll")
    sites = models.ManyToManyField(Site, verbose_name="list of sites")
    place = models.OneToOneField(Place, verbose_name="related place")

Convention is not to capitalize the first letter of the ``verbose_name``.
Django will automatically capitalize the first letter where it needs to.

Relationships
-------------

Clearly, the power of relational databases lies in relating tables to each
other. Django offers ways to define the three most common types of database
relationships: Many-to-one, many-to-many and one-to-one.

Many-to-one relationships
~~~~~~~~~~~~~~~~~~~~~~~~~

To define a many-to-one relationship, use ``ForeignKey``. You use it just like
any other ``Field`` type: by including it as a class attribute of your model.

``ForeignKey`` requires a positional argument: The class to which the model is
related.

For example, if a ``Car`` model has a ``Manufacturer`` -- that is, a
``Manufacturer`` makes multiple cars but each ``Car`` only has one
``Manufacturer`` -- use the following definitions::

    class Manufacturer(models.Model):
        # ...

    class Car(models.Model):
        manufacturer = models.ForeignKey(Manufacturer)
        # ...

To create a recursive relationship -- an object that has a many-to-one
relationship with itself -- use ``models.ForeignKey('self')``.

If you need to create a relationship on a model that has not yet been defined,
you can use the name of the model, rather than the model object itself::

    class Car(models.Model):
        manufacturer = models.ForeignKey('Manufacturer')
        # ...

    class Manufacturer(models.Model):
        # ...

Note, however, that support for strings around model names in ``ForeignKey`` is
quite new, and it can be buggy in some cases.

Behind the scenes, Django appends ``"_id"`` to the field name to create its
database column name. In the above example, the database table for the ``Car``
model will have a ``manufacturer_id`` column. (You can change this explicitly
by specifying ``db_column``; see ``db_column`` below.)  However, your code
should never have to deal with the database column name, unless you write
custom SQL. You'll always deal with the field names of your model object.

It's suggested, but not required, that the name of a ``ForeignKey`` field
(``manufacturer`` in the example above) be the name of the model, lowercase.
You can, of course, call the field whatever you want. For example::

    class Car(models.Model):
        company_that_makes_it = models.ForeignKey(Manufacturer)
        # ...

See the `Many-to-one relationship model example`_ for a full example.

.. _Many-to-one relationship model example: http://www.djangoproject.com/documentation/models/many_to_one/

``ForeignKey`` fields take a number of extra arguments for defining how the
relationship should work. All are optional:

    =======================  ============================================================
    Argument                 Description
    =======================  ============================================================
    ``edit_inline``          If not ``False``, this related object is edited
                             "inline" on the related object's page. This means
                             that the object will not have its own admin
                             interface. Use either ``models.TABULAR`` or ``models.STACKED``,
                             which, respectively, designate whether the inline-editable
                             objects are displayed as a table or as a "stack" of
                             fieldsets.

    ``limit_choices_to``     A dictionary of lookup arguments and values (see
                             the `Database API reference`_) that limit the
                             available admin choices for this object. Use this
                             with ``models.LazyDate`` to limit choices of objects
                             by date. For example::

                                limit_choices_to = {'pub_date__lte': models.LazyDate()}

                             only allows the choice of related objects with a
                             ``pub_date`` before the current date/time to be
                             chosen.

                             Instead of a dictionary this can also be a ``Q`` object
                             (an object with a ``get_sql()`` method) for more complex
                             queries.

                             Not compatible with ``edit_inline``.

    ``max_num_in_admin``     For inline-edited objects, this is the maximum
                             number of related objects to display in the admin.
                             Thus, if a pizza could only have up to 10
                             toppings, ``max_num_in_admin=10`` would ensure
                             that a user never enters more than 10 toppings.

                             Note that this doesn't ensure more than 10 related
                             toppings ever get created. It simply controls the
                             admin interface; it doesn't enforce things at the
                             Python API level or database level.

    ``min_num_in_admin``     The minimum number of related objects displayed in
                             the admin. Normally, at the creation stage,
                             ``num_in_admin`` inline objects are shown, and at
                             the edit stage ``num_extra_on_change`` blank
                             objects are shown in addition to all pre-existing
                             related objects.  However, no fewer than
                             ``min_num_in_admin`` related objects will ever be
                             displayed.

    ``num_extra_on_change``  The number of extra blank related-object fields to
                             show at the change stage.

    ``num_in_admin``         The default number of inline objects to display
                             on the object page at the add stage.

    ``raw_id_admin``         Only display a field for the integer to be entered
                             instead of a drop-down menu. This is useful when
                             related to an object type that will have too many
                             rows to make a select box practical.

                             Not used with ``edit_inline``.

    ``related_name``         The name to use for the relation from the related
                             object back to this one. See the
                             `related objects documentation`_ for a full
                             explanation and example.

    ``to_field``             The field on the related object that the relation
                             is to. By default, Django uses the primary key of
                             the related object.
    =======================  ============================================================

.. _`Database API reference`: http://www.djangoproject.com/documentation/db_api/
.. _related objects documentation: http://www.djangoproject.com/documentation/db_api/#related-objects

Many-to-many relationships
~~~~~~~~~~~~~~~~~~~~~~~~~~

To define a many-to-many relationship, use ``ManyToManyField``. You use it just
like any other ``Field`` type: by including it as a class attribute of your
model.

``ManyToManyField`` requires a positional argument: The class to which the
model is related.

For example, if a ``Pizza`` has multiple ``Topping`` objects -- that is, a
``Topping`` can be on multiple pizzas and each ``Pizza`` has multiple toppings --
here's how you'd represent that::

    class Topping(models.Model):
        # ...

    class Pizza(models.Model):
        # ...
        toppings = models.ManyToManyField(Topping)

As with ``ForeignKey``, a relationship to self can be defined by using the
string ``'self'`` instead of the model name, and you can refer to as-yet
undefined models by using a string containing the model name.

It's suggested, but not required, that the name of a ``ManyToManyField``
(``toppings`` in the example above) be a plural describing the set of related
model objects.

Behind the scenes, Django creates an intermediary join table to represent the
many-to-many relationship.

It doesn't matter which model gets the ``ManyToManyField``, but you only need
it in one of the models -- not in both.

Generally, ``ManyToManyField`` instances should go in the object that's going
to be edited in the admin interface, if you're using Django's admin. In the
above example, ``toppings`` is in ``Pizza`` (rather than ``Topping`` having a
``pizzas`` ``ManyToManyField`` ) because it's more natural to think about a
``Pizza`` having toppings than a topping being on multiple pizzas. The way it's
set up above, the ``Pizza`` admin form would let users select the toppings.

See the `Many-to-many relationship model example`_ for a full example.

.. _Many-to-many relationship model example: http://www.djangoproject.com/documentation/models/many_to_many/

``ManyToManyField`` objects take a number of extra arguments for defining how
the relationship should work. All are optional:

    =======================  ============================================================
    Argument                 Description
    =======================  ============================================================
    ``related_name``         See the description under ``ForeignKey`` above.

    ``filter_interface``     Use a nifty unobtrusive Javascript "filter" interface
                             instead of the usability-challenged ``<select multiple>``
                             in the admin form for this object. The value should be
                             ``models.HORIZONTAL`` or ``models.VERTICAL`` (i.e.
                             should the interface be stacked horizontally or
                             vertically).

    ``limit_choices_to``     See the description under ``ForeignKey`` above.

    ``symmetrical``          Only used in the definition of ManyToManyFields on self.
                             Consider the following model:

                             class Person(models.Model):
                                 friends = models.ManyToManyField("self")

                             When Django processes this model, it identifies that it has
                             a ``ManyToManyField`` on itself, and as a result, it
                             doesn't add a ``person_set`` attribute to the ``Person``
                             class. Instead, the ``ManyToManyField`` is assumed to be
                             symmetrical -- that is, if I am your friend, then you are
                             my friend.

                             If you do not want symmetry in ``ManyToMany`` relationships
                             with ``self``, set ``symmetrical`` to ``False``. This will
                             force Django to add the descriptor for the reverse
                             relationship, allowing ``ManyToMany`` relationships to be
                             non-symmetrical.

    =======================  ============================================================

One-to-one relationships
~~~~~~~~~~~~~~~~~~~~~~~~

The semantics of one-to-one relationships will be changing soon, so we don't
recommend you use them. If that doesn't scare you away, keep reading.

To define a one-to-one relationship, use ``OneToOneField``. You use it just
like any other ``Field`` type: by including it as a class attribute of your
model.

This is most useful on the primary key of an object when that object "extends"
another object in some way.

``OneToOneField`` requires a positional argument: The class to which the
model is related.

For example, if you're building a database of "places", you would build pretty
standard stuff such as address, phone number, etc. in the database. Then, if you
wanted to build a database of restaurants on top of the places, instead of
repeating yourself and replicating those fields in the ``Restaurant`` model, you
could make ``Restaurant`` have a ``OneToOneField`` to ``Place`` (because a
restaurant "is-a" place).

As with ``ForeignKey``, a relationship to self can be defined by using the
string ``"self"`` instead of the model name; references to as-yet undefined
models can be made by using a string containing the model name.

This ``OneToOneField`` will actually replace the primary key ``id`` field
(since one-to-one relations share the same primary key), and will be displayed
as a read-only field when you edit an object in the admin interface:

See the `One-to-one relationship model example`_ for a full example.

.. _One-to-one relationship model example: http://www.djangoproject.com/documentation/models/one_to_one/

Meta options
============

Give your model metadata by using an inner ``class Meta``, like so::

    class Foo(models.Model):
        bar = models.CharField(maxlength=30)

        class Meta:
            # ...

Model metadata is "anything that's not a field", such as ordering options, etc.

Here's a list of all possible ``Meta`` options. No options are required. Adding
``class Meta`` to a model is completely optional.

``db_table``
------------

The name of the database table to use for the model::

    db_table = 'music_album'

If this isn't given, Django will use ``app_label + '_' + model_class_name``.
See "Table names" below for more.

If your database table name is an SQL reserved word, or contains characters
that aren't allowed in Python variable names -- notably, the hyphen --
that's OK. Django quotes column and table names behind the scenes.

``get_latest_by``
-----------------

The name of a ``DateField`` or ``DateTimeField`` in the model. This specifies
the default field to use in your model ``Manager``'s ``latest()`` method.

Example::

    get_latest_by = "order_date"

See the `docs for latest()`_ for more.

.. _docs for latest(): http://www.djangoproject.com/documentation/db_api/#latest-field-name-none

``order_with_respect_to``
-------------------------

Marks this object as "orderable" with respect to the given field. This is
almost always used with related objects to allow them to be ordered with
respect to a parent object. For example, if an ``Answer`` relates to a
``Question`` object, and a question has more than one answer, and the order
of answers matters, you'd do this::

    class Answer(models.Model):
        question = models.ForeignKey(Question)
        # ...

        class Meta:
            order_with_respect_to = 'question'

``ordering``
------------

The default ordering for the object, for use when obtaining lists of objects::

    ordering = ['-order_date']

This is a tuple or list of strings. Each string is a field name with an
optional "-" prefix, which indicates descending order. Fields without a
leading "-" will be ordered ascending. Use the string "?" to order randomly.

For example, to order by a ``pub_date`` field ascending, use this::

    ordering = ['pub_date']

To order by ``pub_date`` descending, use this::

    ordering = ['-pub_date']

To order by ``pub_date`` descending, then by ``author`` ascending, use this::

    ordering = ['-pub_date', 'author']

See `Specifying ordering`_ for more examples.

Note that, regardless of how many fields are in ``ordering``, the admin
site uses only the first field.

.. _Specifying ordering: http://www.djangoproject.com/documentation/models/ordering/

``permissions``
---------------

Extra permissions to enter into the permissions table when creating this
object. Add, delete and change permissions are automatically created for
each object that has ``admin`` set. This example specifies an extra
permission, ``can_deliver_pizzas``::

    permissions = (("can_deliver_pizzas", "Can deliver pizzas"),)

This is a list or tuple of 2-tuples in the format
``(permission_code, human_readable_permission_name)``.

``unique_together``
-------------------

Sets of field names that, taken together, must be unique::

    unique_together = (("driver", "restaurant"),)

This is a list of lists of fields that must be unique when considered
together. It's used in the Django admin and is enforced at the database
level (i.e., the appropriate ``UNIQUE`` statements are included in the
``CREATE TABLE`` statement).

``verbose_name``
----------------

A human-readable name for the object, singular::

    verbose_name = "pizza"

If this isn't given, Django will use a munged version of the class name:
``CamelCase`` becomes ``camel case``.

``verbose_name_plural``
-----------------------

The plural name for the object::

    verbose_name_plural = "stories"

If this isn't given, Django will use ``verbose_name + "s"``.

Table names
===========

To save you time, Django automatically derives the name of the database table
from the name of your model class and the app that contains it. A model's
database table name is constructed by joining the model's "app label" -- the
name you used in ``manage.py startapp`` -- to the model's class name, with an
underscore between them.

For example, if you have an app ``bookstore`` (as created by
``manage.py startapp bookstore``), a model defined as ``class Book`` will have
a database table named ``bookstore_book``.

To override the database table name, use the ``db_table`` parameter in
``class Meta``.

Automatic primary key fields
============================

By default, Django gives each model the following field::

    id = models.AutoField(primary_key=True)

This is an auto-incrementing primary key.

If you'd like to specify a custom primary key, just specify ``primary_key=True``
on one of your fields. If Django sees you've explicitly set ``primary_key``, it
won't add the automatic ``id`` column.

Each model requires exactly one field to have ``primary_key=True``.

Admin options
=============

If you want your model to be visible to Django's admin site, give your model an
inner ``"class Admin"``, like so::

    class Person(models.Model):
        first_name = models.CharField(maxlength=30)
        last_name = models.CharField(maxlength=30)

        class Admin:
            # Admin options go here
            pass

The ``Admin`` class tells Django how to display the model in the admin site.

Here's a list of all possible ``Admin`` options. None of these options are
required. To use an admin interface without specifying any options, use
``pass``, like so::

    class Admin:
        pass

Adding ``class Admin`` to a model is completely optional.

``date_hierarchy``
------------------

Set ``date_hierarchy`` to the name of a ``DateField`` or ``DateTimeField`` in
your model, and the change list page will include a date-based drilldown
navigation by that field.

Example::

    date_hierarchy = 'pub_date'

``fields``
----------

Set ``fields`` to control the layout of admin "add" and "change" pages.

``fields`` is a list of two-tuples, in which each two-tuple represents a
``<fieldset>`` on the admin form page. (A ``<fieldset>`` is a "section" of the
form.)

The two-tuples are in the format ``(name, field_options)``, where ``name`` is a
string representing the title of the fieldset and ``field_options`` is a
dictionary of information about the fieldset, including a list of fields to be
displayed in it.

A full example, taken from the ``django.contrib.flatpages.FlatPage`` model::

    class Admin:
        fields = (
            (None, {
                'fields': ('url', 'title', 'content', 'sites')
            }),
            ('Advanced options', {
                'classes': 'collapse',
                'fields' : ('enable_comments', 'registration_required', 'template_name')
            }),
        )

This results in an admin page that looks like:

    .. image:: http://media.djangoproject.com/img/doc/flatfiles_admin.png

If ``fields`` isn't given, Django will default to displaying each field that
isn't an ``AutoField`` and has ``editable=True``, in a single fieldset, in
the same order as the fields are defined in the model.

The ``field_options`` dictionary can have the following keys:

``fields``
~~~~~~~~~~

A tuple of field names to display in this fieldset. This key is required.

Example::

    {
    'fields': ('first_name', 'last_name', 'address', 'city', 'state'),
    }

To display multiple fields on the same line, wrap those fields in their own
tuple. In this example, the ``first_name`` and ``last_name`` fields will
display on the same line::

    {
    'fields': (('first_name', 'last_name'), 'address', 'city', 'state'),
    }

``classes``
~~~~~~~~~~~

A string containing extra CSS classes to apply to the fieldset.

Example::

    {
    'classes': 'wide',
    }

Apply multiple classes by separating them with spaces. Example::

    {
    'classes': 'wide extrapretty',
    }

Two useful classes defined by the default admin-site stylesheet are
``collapse`` and ``wide``. Fieldsets with the ``collapse`` style will be
initially collapsed in the admin and replaced with a small "click to expand"
link. Fieldsets with the ``wide`` style will be given extra horizontal space.

``description``
~~~~~~~~~~~~~~~

A string of optional extra text to be displayed at the top of each fieldset,
under the heading of the fieldset. It's used verbatim, so you can use any HTML
and you must escape any special HTML characters (such as ampersands) yourself.

``js``
------

A list of strings representing URLs of JavaScript files to link into the admin
screen via ``<script src="">`` tags. This can be used to tweak a given type of
admin page in JavaScript or to provide "quick links" to fill in default values
for certain fields.

``list_display``
----------------

Set ``list_display`` to control which fields are displayed on the change list
page of the admin.

Example::

    list_display = ('first_name', 'last_name')

If you don't set ``list_display``, the admin site will display a single column
that displays the ``__str__()`` representation of each object.

A few special cases to note about ``list_display``:

    * If the field is a ``ForeignKey``, Django will display the ``__str__()``
      of the related object.

    * ``ManyToManyField`` fields aren't supported, because that would entail
        executing a separate SQL statement for each row in the table.

    * If the field is a ``BooleanField``, Django will display a pretty "on" or
      "off" icon instead of ``True`` or ``False``.

    * If the string given is a method of the model, Django will call it and
      display the output. This method should have a ``short_description``
      function attribute, for use as the header for the field.

      Here's a full example model::

          class Person(models.Model):
              name = models.CharField(maxlength=50)
              birthday = models.DateField()

              class Admin:
                  list_display = ('name', 'decade_born_in')

              def decade_born_in(self):
                  return self.birthday.strftime('%Y')[:3] + "0's"
              decade_born_in.short_description = 'Birth decade'

    * If the string given is a method of the model, Django will HTML-escape the
      output by default. If you'd rather not escape the output of the method,
      give the method an ``allow_tags`` attribute whose value is ``True``.

      Here's a full example model::

          class Person(models.Model):
              first_name = models.CharField(maxlength=50)
              last_name = models.CharField(maxlength=50)
              color_code = models.CharField(maxlength=6)

              class Admin:
                  list_display = ('first_name', 'last_name', 'colored_name')

              def colored_name(self):
                  return '<span style="color: #%s;">%s %s</span>' % (self.color_code, self.first_name, self.last_name)
              colored_name.allow_tags = True

``list_display_links``
----------------------

Set ``list_display_links`` to control which fields in ``list_display`` should
be linked to the "change" page for an object.

By default, the change list page will link the first column -- the first field
specified in ``list_display`` -- to the change page for each item. But
``list_display_links`` lets you change which columns are linked. Set
``list_display_links`` to a list or tuple of field names (in the same format as
``list_display``) to link.

``list_display_links`` can specify one or many field names. As long as the
field names appear in ``list_display``, Django doesn't care how many (or how
few) fields are linked. The only requirement is: If you want to use
``list_display_links``, you must define ``list_display``.

In this example, the ``first_name`` and ``last_name`` fields will be linked on
the change list page::

    class Admin:
        list_display = ('first_name', 'last_name', 'birthday')
        list_display_links = ('first_name', 'last_name')

Finally, note that in order to use ``list_display_links``, you must define
``list_display``, too.

``list_filter``
---------------

Set ``list_filter`` to activate filters in the right sidebar of the change list
page of the admin. This should be a list of field names, and each specified
field should be either a ``BooleanField``, ``DateField``, ``DateTimeField``
or ``ForeignKey``.

This example, taken from the ``django.contrib.auth.models.User`` model, shows
how both ``list_display`` and ``list_filter`` work::

    class Admin:
        list_display = ('username', 'email', 'first_name', 'last_name', 'is_staff')
        list_filter = ('is_staff', 'is_superuser')

The above code results in an admin change list page that looks like this:

    .. image:: http://media.djangoproject.com/img/doc/users_changelist.png

(This example also has ``search_fields`` defined. See below.)

``list_per_page``
-----------------

Set ``list_per_page`` to control how many items appear on each paginated admin
change list page. By default, this is set to ``100``.

``list_select_related``
-----------------------

Set ``list_select_related`` to tell Django to use ``select_related()`` in
retrieving the list of objects on the admin change list page. This can save you
a bunch of database queries.

The value should be either ``True`` or ``False``. Default is ``False``.

Note that Django will use ``select_related()``, regardless of this setting,
if one of the ``list_display`` fields is a ``ForeignKey``.

For more on ``select_related()``, see `the select_related() docs`_.

.. _the select_related() docs: http://www.djangoproject.com/documentation/db_api/#select-related

``ordering``
------------

Set ``ordering`` to specify how objects on the admin change list page should be
ordered. This should be a list or tuple in the same format as a model's
``ordering`` parameter.

If this isn't provided, the Django admin will use the model's default ordering.

``save_as``
-----------

Set ``save_as`` to enable a "save as" feature on admin change forms.

Normally, objects have three save options: "Save", "Save and continue editing"
and "Save and add another". If ``save_as`` is ``True``, "Save and add another"
will be replaced by a "Save as" button.

"Save as" means the object will be saved as a new object (with a new ID),
rather than the old object.

By default, ``save_as`` is set to ``False``.

``save_on_top``
---------------

Set ``save_on_top`` to add save buttons across the top of your admin change
forms.

Normally, the save buttons appear only at the bottom of the forms. If you set
``save_on_top``, the buttons will appear both on the top and the bottom.

By default, ``save_on_top`` is set to ``False``.

``search_fields``
-----------------

Set ``search_fields`` to enable a search box on the admin change list page.
This should be set to a list of field names that will be searched whenever
somebody submits a search query in that text box.

These fields should be some kind of text field, such as ``CharField`` or
``TextField``.

When somebody does a search in the admin search box, Django splits the search
query into words and returns all objects that contain each of the words, case
insensitive, where each word must be in at least one of ``search_fields``. For
example, if ``search_fields`` is set to ``['first_name', 'last_name']`` and a
user searches for ``john lennon``, Django will do the equivalent of this SQL
``WHERE`` clause::

    WHERE (first_name ILIKE '%john%' OR last_name ILIKE '%john%')
    AND (first_name ILIKE '%lennon%' OR last_name ILIKE '%lennon%')

Managers
========

A ``Manager`` is the interface through which database query operations are
provided to Django models. At least one ``Manager`` exists for every model in
a Django application.

The way ``Manager`` classes work is documented in the `Retrieving objects`_
section of the database API docs, but this section specifically touches on
model options that customize ``Manager`` behavior.

.. _Retrieving objects: http://www.djangoproject.com/documentation/db_api/#retrieving-objects

Manager names
-------------

By default, Django adds a ``Manager`` with the name ``objects`` to every Django
model class. However, if you want to use ``objects`` as a field name, or if you
want to use a name other than ``objects`` for the ``Manager``, you can rename
it on a per-model basis. To rename the ``Manager`` for a given class, define a
class attribute of type ``models.Manager()`` on that model. For example::

    from django.db import models

    class Person(models.Model):
        #...
        people = models.Manager()

Using this example model, ``Person.objects`` will generate an
``AttributeError`` exception, but ``Person.people.all()`` will provide a list
of all ``Person`` objects.

Custom Managers
---------------

You can use a custom ``Manager`` in a particular model by extending the base
``Manager`` class and instantiating your custom ``Manager`` in your model.

There are two reasons you might want to customize a ``Manager``: to add extra
``Manager`` methods, and/or to modify the initial ``QuerySet`` the ``Manager``
returns.

Adding extra Manager methods
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Adding extra ``Manager`` methods is the preferred way to add "table-level"
functionality to your models. (For "row-level" functionality -- i.e., functions
that act on a single instance of a model object -- use _`Model methods`, not
custom ``Manager`` methods.)

A custom ``Manager`` method can return anything you want. It doesn't have to
return a ``QuerySet``.

For example, this custom ``Manager`` offers a method ``with_counts()``, which
returns a list of all ``OpinionPoll`` objects, each with an extra
``num_responses`` attribute that is the result of an aggregate query::

    class PollManager(models.Manager):
        def with_counts(self):
            from django.db import connection
            cursor = connection.cursor()
            cursor.execute("""
                SELECT p.id, p.question, p.poll_date, COUNT(*)
                FROM polls_opinionpoll p, polls_response r
                WHERE p.id = r.poll_id
                GROUP BY 1, 2, 3
                ORDER BY 3 DESC""")
            result_list = []
            for row in cursor.fetchall():
                p = self.model(id=row[0], question=row[1], poll_date=row[2])
                p.num_responses = row[3]
                result_list.append(p)
            return result_list

    class OpinionPoll(models.Model):
        question = models.CharField(maxlength=200)
        poll_date = models.DateField()
        objects = PollManager()

    class Response(models.Model):
        poll = models.ForeignKey(Poll)
        person_name = models.CharField(maxlength=50)
        response = models.TextField()

With this example, you'd use ``OpinionPoll.objects.with_counts()`` to return
that list of ``OpinionPoll`` objects with ``num_responses`` attributes.

Another thing to note about this example is that ``Manager`` methods can
access ``self.model`` to get the model class to which they're attached.

Modifying initial Manager QuerySets
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

A ``Manager``'s base ``QuerySet`` returns all objects in the system. For
example, using this model::

    class Book(models.Model):
        title = models.CharField(maxlength=100)
        author = models.CharField(maxlength=50)

...the statement ``Book.objects.all()`` will return all books in the database.

You can override a ``Manager``\'s base ``QuerySet`` by overriding the
``Manager.get_query_set()`` method. ``get_query_set()`` should return a
``QuerySet`` with the properties you require.

For example, the following model has *two* ``Manager``\s -- one that returns
all objects, and one that returns only the books by Roald Dahl::

    # First, define the Manager subclass.
    class DahlBookManager(models.Manager):
        def get_query_set(self):
            return super(DahlBookManager, self).get_query_set().filter(author='Roald Dahl')

    # Then hook it into the Book model explicitly.
    class Book(models.Model):
        title = models.CharField(maxlength=100)
        author = models.CharField(maxlength=50)

        objects = models.Manager() # The default manager.
        dahl_objects = DahlBookManager() # The Dahl-specific manager.

With this sample model, ``Book.objects.all()`` will return all books in the
database, but ``Book.dahl_objects.all()`` will only return the ones written by
Roald Dahl.

Of course, because ``get_query_set()`` returns a ``QuerySet`` object, you can
use ``filter()``, ``exclude()`` and all the other ``QuerySet`` methods on it.
So these statements are all legal::

    Book.dahl_objects.all()
    Book.dahl_objects.filter(title='Matilda')
    Book.dahl_objects.count()

This example also pointed out another interesting technique: using multiple
managers on the same model. You can attach as many ``Manager()`` instances to
a model as you'd like. This is an easy way to define common "filters" for your
models.

For example::

    class MaleManager(models.Manager):
        def get_query_set(self):
            return super(MaleManager, self).get_query_set().filter(sex='M')

    class FemaleManager(models.Manager):
        def get_query_set(self):
            return super(FemaleManager, self).get_query_set().filter(sex='F')

    class Person(models.Model):
        first_name = models.CharField(maxlength=50)
        last_name = models.CharField(maxlength=50)
        sex = models.CharField(maxlength=1, choices=(('M', 'Male'), ('F', 'Female')))
        people = models.Manager()
        men = MaleManager()
        women = FemaleManager()

This example allows you to request ``Person.men.all()``, ``Person.women.all()``,
and ``Person.people.all()``, yielding predictable results.

If you use custom ``Manager`` objects, take note that the first ``Manager``
Django encounters (in order by which they're defined in the model) has a
special status. Django interprets the first ``Manager`` defined in a class as
the "default" ``Manager``. Certain operations -- such as Django's admin site --
use the default ``Manager`` to obtain lists of objects, so it's generally a
good idea for the first ``Manager`` to be relatively unfiltered. In the last
example, the ``people`` ``Manager`` is defined first -- so it's the default
``Manager``.

Model methods
=============

Define custom methods on a model to add custom "row-level" functionality to
your objects. Whereas ``Manager`` methods are intended to do "table-wide"
things, model methods should act on a particular model instance.

This is a valuable technique for keeping business logic in one place -- the
model.

For example, this model has a few custom methods::

    class Person(models.Model):
        first_name = models.CharField(maxlength=50)
        last_name = models.CharField(maxlength=50)
        birth_date = models.DateField()
        address = models.CharField(maxlength=100)
        city = models.CharField(maxlength=50)
        state = models.USStateField() # Yes, this is America-centric...

        def baby_boomer_status(self):
            "Returns the person's baby-boomer status."
            import datetime
            if datetime.date(1945, 8, 1) <= self.birth_date <= datetime.date(1964, 12, 31):
                return "Baby boomer"
            if self.birth_date < datetime.date(1945, 8, 1):
                return "Pre-boomer"
            return "Post-boomer"

        def is_midwestern(self):
            "Returns True if this person is from the Midwest."
            return self.state in ('IL', 'WI', 'MI', 'IN', 'OH', 'IA', 'MO')

        def _get_full_name(self):
            "Returns the person's full name."
            return '%s %s' % (self.first_name, self.last_name)
        full_name = property(_get_full_name)

The last method in this example is a *property*. `Read more about properties`_.

.. _Read more about properties: http://www.python.org/download/releases/2.2/descrintro/#property

A few object methods have special meaning:

``__str__``
-----------

``__str__()`` is a Python "magic method" that defines what should be returned
if you call ``str()`` on the object. Django uses ``str(obj)`` in a number of
places, most notably as the value displayed to render an object in the Django
admin site and as the value inserted into a template when it displays an
object. Thus, you should always return a nice, human-readable string for the
object's ``__str__``. Although this isn't required, it's strongly encouraged.

For example::

    class Person(models.Model):
        first_name = models.CharField(maxlength=50)
        last_name = models.CharField(maxlength=50)

        def __str__(self):
            return '%s %s' % (self.first_name, self.last_name)

``get_absolute_url``
--------------------

Define a ``get_absolute_url()`` method to tell Django how to calculate the
URL for an object. For example::

    def get_absolute_url(self):
        return "/people/%i/" % self.id

Django uses this in its admin interface. If an object defines
``get_absolute_url()``, the object-editing page will have a "View on site"
link that will jump you directly to the object's public view, according to
``get_absolute_url()``.

Also, a couple of other bits of Django, such as the syndication-feed framework,
use ``get_absolute_url()`` as a convenience to reward people who've defined the
method.

It's good practice to use ``get_absolute_url()`` in templates, instead of
hard-coding your objects' URLs. For example, this template code is bad::

    <a href="/people/{{ object.id }}/">{{ object.name }}</a>

But this template code is good::

    <a href="{{ object.get_absolute_url }}">{{ object.name }}</a>

(Yes, we know ``get_absolute_url()`` couples URLs to models, which violates the
DRY principle, because URLs are defined both in a URLconf and in the model.
This is a rare case in which we've intentionally violated that principle for
the sake of convenience. With that said, we're working on an even cleaner way
of specifying URLs in a more DRY fashion.)

Executing custom SQL
--------------------

Feel free to write custom SQL statements in custom model methods and
module-level methods. The object ``django.db.connection`` represents the
current database connection. To use it, call ``connection.cursor()`` to get a
cursor object. Then, call ``cursor.execute(sql, [params])`` to execute the SQL
and ``cursor.fetchone()`` or ``cursor.fetchall()`` to return the resulting
rows. Example::

    def my_custom_sql(self):
        from django.db import connection
        cursor = connection.cursor()
        cursor.execute("SELECT foo FROM bar WHERE baz = %s", [self.baz])
        row = cursor.fetchone()
        return row

``connection`` and ``cursor`` simply use the standard `Python DB-API`_. If
you're not familiar with the Python DB-API, note that the SQL statement in
``cursor.execute()`` uses placeholders, ``"%s"``, rather than adding parameters
directly within the SQL. If you use this technique, the underlying database
library will automatically add quotes and escaping to your parameter(s) as
necessary. (Also note that Django expects the ``"%s"`` placeholder, *not* the
``"?"`` placeholder, which is used by the SQLite Python bindings. This is for
the sake of consistency and sanity.)

A final note: If all you want to do is a custom ``WHERE`` clause, you can just
just the ``where``, ``tables`` and ``params`` arguments to the standard lookup
API. See `Other lookup options`_.

.. _Python DB-API: http://www.python.org/peps/pep-0249.html
.. _Other lookup options: http://www.djangoproject.com/documentation/db_api/#extra-params-select-where-tables

Overriding default model methods
--------------------------------

As explained in the `database API docs`_, each model gets a few methods
automatically -- most notably, ``save()`` and ``delete()``. You can override
these methods to alter behavior.

A classic use-case for overriding the built-in methods is if you want something
to happen whenever you save an object. For example::

    class Blog(models.Model):
        name = models.CharField(maxlength=100)
        tagline = models.TextField()

        def save(self):
            do_something()
            super(Blog, self).save() # Call the "real" save() method.
            do_something_else()

You can also prevent saving::

    class Blog(models.Model):
        name = models.CharField(maxlength=100)
        tagline = models.TextField()

        def save(self):
            if self.name == "Yoko Ono's blog":
                return # Yoko shall never have her own blog!
            else:
                super(Blog, self).save() # Call the "real" save() method.

.. _database API docs: http://www.djangoproject.com/documentation/db_api/

Models across files
===================

It's perfectly OK to relate a model to one from another app. To do this, just
import the related model at the top of the model that holds your model. Then,
just refer to the other model class wherever needed. For example::

    from mysite.geography.models import ZipCode

    class Restaurant(models.Model):
        # ...
        zip_code = models.ForeignKey(ZipCode)

Using models
============

Once you have created your models, the final step is to tell Django you're
going to *use* those models.

Do this by editing your settings file and changing the ``INSTALLED_APPS``
setting to add the name of the module that contains your ``models.py``.

For example, if the models for your application live in the module
``mysite.myapp.models`` (the package structure that is created for an
application by the ``manage.py startapp`` script), ``INSTALLED_APPS`` should
read, in part::

    INSTALLED_APPS = (
        #...
        'mysite.myapp',
        #...
    )

Providing initial SQL data
==========================

Django provides a hook for passing the database arbitrary SQL that's executed
just after the CREATE TABLE statements. Use this hook, for example, if you want
to populate default records, or create SQL functions, automatically.

The hook is simple: Django just looks for a file called
``<appname>/sql/<modelname>.sql``, where ``<appname>`` is your app directory and
``<modelname>`` is the model's name in lowercase.

In the ``Person`` example model at the top of this document, assuming it lives
in an app called ``myapp``, you could add arbitrary SQL to the file
``myapp/sql/person.sql``. Here's an example of what the file might contain::

    INSERT INTO myapp_person (first_name, last_name) VALUES ('John', 'Lennon');
    INSERT INTO myapp_person (first_name, last_name) VALUES ('Paul', 'McCartney');

Each SQL file, if given, is expected to contain valid SQL. The SQL files are
piped directly into the database after all of the models' table-creation
statements have been executed.

The SQL files are read by the ``sqlinitialdata``, ``sqlreset``, ``sqlall`` and
``reset`` commands in ``manage.py``. Refer to the `manage.py documentation`_
for more information.

Note that if you have multiple SQL data files, there's no guarantee of the
order in which they're executed. The only thing you can assume is that, by the
time your custom data files are executed, all the database tables already will
have been created.

.. _`manage.py documentation`: http://www.djangoproject.com/documentation/django_admin/#sqlinitialdata-appname-appname

Database-backend-specific SQL data
----------------------------------

There's also a hook for backend-specific SQL data. For example, you can have
separate initial-data files for PostgreSQL and MySQL. For each app, Django
looks for a file called ``<appname>/sql/<modelname>.<backend>.sql``, where
``<appname>`` is your app directory, ``<modelname>`` is the model's name in
lowercase and ``<backend>`` is the value of ``DATABASE_ENGINE`` in your
settings file (e.g., ``postgresql``, ``mysql``).

Backend-specific SQL data is executed before non-backend-specific SQL data. For
example, if your app contains the files ``sql/person.sql`` and
``sql/person.postgresql.sql`` and you're installing the app on PostgreSQL,
Django will execute the contents of ``sql/person.postgresql.sql`` first, then
``sql/person.sql``.