======

Models

======

.. module:: django.db.models

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

      :class:`django.db.models.Model`.

    * Each attribute of the model represents a database field.

    * With all of this, Django gives you an automatically-generated

      database-access API; see :doc:`/topics/db/queries`.

.. seealso::

    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.)

    .. _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(max_length=30)

        last_name = models.CharField(max_length=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:

.. code-block:: sql

    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 :ref:`table-names` for more

      details..

    * An ``id`` field is added automatically, but this behavior can be

      overridden. See :ref:`automatic-primary-key-fields`.

    * 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 :doc:`settings file </topics/settings>`.

Using models

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

Once you have defined your models, you need to tell Django you're going to *use*

those models. Do this by editing your settings file and changing the

:setting:`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 :djadmin:`manage.py startapp <startapp>` script),

:setting:`INSTALLED_APPS` should read, in part::

    INSTALLED_APPS = (

        #...

        'mysite.myapp',

        #...

    )

When you add new apps to :setting:`INSTALLED_APPS`, be sure to run

:djadmin:`manage.py syncdb <syncdb>`.

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(max_length=50)

        last_name = models.CharField(max_length=50)

        instrument = models.CharField(max_length=100)

    class Album(models.Model):

        artist = models.ForeignKey(Musician)

        name = models.CharField(max_length=100)

        release_date = models.DateField()

        num_stars = models.IntegerField()

Field types

-----------

Each field in your model should be an instance of the appropriate

:class:`~django.db.models.Field` class. Django uses the field class types to

determine a few things:

    * The database column type (e.g. ``INTEGER``, ``VARCHAR``).

    * The :doc:`widget </ref/forms/widgets>` 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

      automatically-generated forms.

Django ships with dozens of built-in field types; you can find the complete list

in the :ref:`model field reference <model-field-types>`. You can easily write

your own fields if Django's built-in ones don't do the trick; see

:doc:`/howto/custom-model-fields`.

Field options

-------------

Each field takes a certain set of field-specific arguments (documented in the

:ref:`model field reference <model-field-types>`). For example,

:class:`~django.db.models.CharField` (and its subclasses) require a

:attr:`~django.db.models.CharField.max_length` argument which specifies the size

of the ``VARCHAR`` database field used to store the data.

There's also a set of common arguments available to all field types. All are

optional. They're fully explained in the :ref:`reference

<common-model-field-options>`, but here's a quick summary of the most often-used

ones:

    :attr:`~Field.null`

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

        Default is ``False``.

    :attr:`~Field.blank`

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

        Note that this is different than :attr:`~Field.null`.

        :attr:`~Field.null` is purely database-related, whereas

        :attr:`~Field.blank` is validation-related. If a field has

        :attr:`blank=True <Field.blank>`, validation on Django's admin site will

        allow entry of an empty value. If a field has :attr:`blank=False

        <Field.blank>`, the field will be required.

    :attr:`~Field.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 = (

                (u'FR', u'Freshman'),

                (u'SO', u'Sophomore'),

                (u'JR', u'Junior'),

                (u'SR', u'Senior'),

                (u'GR', u'Graduate'),

            )

        The first element in each tuple is the value that will be stored in the

        database, the second element will be displayed by the admin interface,

        or in a ModelChoiceField. Given an instance of a model object, the

        display value for a choices field can be accessed using the

        ``get_FOO_display`` method. For example::

            from django.db import models

            class Person(models.Model):

                GENDER_CHOICES = (

                    (u'M', u'Male'),

                    (u'F', u'Female'),

                )

                name = models.CharField(max_length=60)

                gender = models.CharField(max_length=2, choices=GENDER_CHOICES)

        ::

            >>> p = Person(name="Fred Flinstone", gender="M")

            >>> p.save()

            >>> p.gender

            u'M'

            >>> p.get_gender_display()

            u'Male'

    :attr:`~Field.default`

        The default value for the field. This can be a value or a callable

        object. If callable it will be called every time a new object is

        created.

    :attr:`~Field.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.

    :attr:`~Field.primary_key`

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

        If you don't specify :attr:`primary_key=True <Field.primary_key>` for

        any fields in your model, Django will automatically add an

        :class:`IntegerField` to hold the primary key, so you don't need to set

        :attr:`primary_key=True <Field.primary_key>` on any of your fields

        unless you want to override the default primary-key behavior. For more,

        see :ref:`automatic-primary-key-fields`.

    :attr:`~Field.unique`

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

Again, these are just short descriptions of the most common field options. Full

details can be found in the :ref:`common model field option reference

<common-model-field-options>`.

.. _automatic-primary-key-fields:

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

:attr:`primary_key=True <Field.primary_key>` on one of your fields. If Django

sees you've explicitly set :attr:`Field.primary_key`, it won't add the automatic

``id`` column.

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

<Field.primary_key>`.

.. _verbose-field-names:

Verbose field names

-------------------

Each field type, except for :class:`~django.db.models.ForeignKey`,

:class:`~django.db.models.ManyToManyField` and

:class:`~django.db.models.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", max_length=30)

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

    first_name = models.CharField(max_length=30)

:class:`~django.db.models.ForeignKey`,

:class:`~django.db.models.ManyToManyField` and

:class:`~django.db.models.OneToOneField` require the first argument to be a

model class, so use the :attr:`~Field.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")

The convention is not to capitalize the first letter of the

:attr:`~Field.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 :class:`~django.db.models.ForeignKey`.

You use it just like any other :class:`~django.db.models.Field` type: by

including it as a class attribute of your model.

:class:`~django.db.models.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)

        # ...

You can also create :ref:`recursive relationships <recursive-relationships>` (an

object with a many-to-one relationship to itself) and :ref:`relationships to

models not yet defined <lazy-relationships>`; see :ref:`the model field

reference <ref-foreignkey>` for details.

It's suggested, but not required, that the name of a

:class:`~django.db.models.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)

        # ...

.. seealso::

    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/

:class:`~django.db.models.ForeignKey` fields also accept a number of extra

arguments which are explained in :ref:`the model field reference

<foreign-key-arguments>`. These options help define how the relationship should

work; all are optional.

Many-to-many relationships

~~~~~~~~~~~~~~~~~~~~~~~~~~

To define a many-to-many relationship, use

:class:`~django.db.models.ManyToManyField`. You use it just like any other

:class:`~django.db.models.Field` type: by including it as a class attribute of

your model.

:class:`~django.db.models.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 :class:`~django.db.models.ForeignKey`, you can also create

:ref:`recursive relationships <recursive-relationships>` (an object with a

many-to-many relationship to itself) and :ref:`relationships to models not yet

defined <lazy-relationships>`; see :ref:`the model field reference

<ref-manytomany>` for details.

It's suggested, but not required, that the name of a

:class:`~django.db.models.ManyToManyField` (``toppings`` in the example above)

be a plural describing the set of related model objects.

It doesn't matter which model gets the

:class:`~django.db.models.ManyToManyField`, but you only need it in one of the

models -- not in both.

Generally, :class:`~django.db.models.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`` :class:`~django.db.models.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.

.. seealso::

    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/

:class:`~django.db.models.ManyToManyField` fields also accept a number of extra

arguments which are explained in :ref:`the model field reference

<manytomany-arguments>`. These options help define how the relationship should

work; all are optional.

.. _intermediary-manytomany:

Extra fields on many-to-many relationships

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. versionadded:: 1.0

When you're only dealing with simple many-to-many relationships such as

mixing and matching pizzas and toppings, a standard :class:`~django.db.models.ManyToManyField` is all you need. However, sometimes

you may need to associate data with the relationship between two models.

For example, consider the case of an application tracking the musical groups

which musicians belong to. There is a many-to-many relationship between a person

and the groups of which they are a member, so you could use a

:class:`~django.db.models.ManyToManyField` to represent this relationship.

However, there is a lot of detail about the membership that you might want to

collect, such as the date at which the person joined the group.

For these situations, Django allows you to specify the model that will be used

to govern the many-to-many relationship. You can then put extra fields on the

intermediate model. The intermediate model is associated with the

:class:`~django.db.models.ManyToManyField` using the

:attr:`through <ManyToManyField.through>` argument to point to the model

that will act as an intermediary. For our musician example, the code would look

something like this::

    class Person(models.Model):

        name = models.CharField(max_length=128)

        def __unicode__(self):

            return self.name

    class Group(models.Model):

        name = models.CharField(max_length=128)

        members = models.ManyToManyField(Person, through='Membership')

        def __unicode__(self):

            return self.name

    class Membership(models.Model):

        person = models.ForeignKey(Person)

        group = models.ForeignKey(Group)

        date_joined = models.DateField()

        invite_reason = models.CharField(max_length=64)

When you set up the intermediary model, you explicitly specify foreign

keys to the models that are involved in the ManyToMany relation. This

explicit declaration defines how the two models are related.

There are a few restrictions on the intermediate model:

    * Your intermediate model must contain one - and *only* one - foreign key

      to the target model (this would be ``Person`` in our example). If you

      have more than one foreign key, a validation error will be raised.

    * Your intermediate model must contain one - and *only* one - foreign key

      to the source model (this would be ``Group`` in our example). If you

      have more than one foreign key, a validation error will be raised.

    * The only exception to this is a model which has a many-to-many

      relationship to itself, through an intermediary model. In this

      case, two foreign keys to the same model are permitted, but they

      will be treated as the two (different) sides of the many-to-many

      relation.

    * When defining a many-to-many relationship from a model to

      itself, using an intermediary model, you *must* use

      :attr:`symmetrical=False <ManyToManyField.symmetrical>` (see

      :ref:`the model field reference <manytomany-arguments>`).

Now that you have set up your :class:`~django.db.models.ManyToManyField` to use

your intermediary model (``Membership``, in this case), you're ready to start

creating some many-to-many relationships. You do this by creating instances of

the intermediate model::

    >>> ringo = Person.objects.create(name="Ringo Starr")

    >>> paul = Person.objects.create(name="Paul McCartney")

    >>> beatles = Group.objects.create(name="The Beatles")

    >>> m1 = Membership(person=ringo, group=beatles,

    ...     date_joined=date(1962, 8, 16),

    ...     invite_reason= "Needed a new drummer.")

    >>> m1.save()

    >>> beatles.members.all()

    [<Person: Ringo Starr>]

    >>> ringo.group_set.all()

    [<Group: The Beatles>]

    >>> m2 = Membership.objects.create(person=paul, group=beatles,

    ...     date_joined=date(1960, 8, 1),

    ...     invite_reason= "Wanted to form a band.")

    >>> beatles.members.all()

    [<Person: Ringo Starr>, <Person: Paul McCartney>]

Unlike normal many-to-many fields, you *can't* use ``add``, ``create``,

or assignment (i.e., ``beatles.members = [...]``) to create relationships::

    # THIS WILL NOT WORK

    >>> beatles.members.add(john)

    # NEITHER WILL THIS

    >>> beatles.members.create(name="George Harrison")

    # AND NEITHER WILL THIS

    >>> beatles.members = [john, paul, ringo, george]

Why? You can't just create a relationship between a ``Person`` and a ``Group``

- you need to specify all the detail for the relationship required by the

``Membership`` model. The simple ``add``, ``create`` and assignment calls

don't provide a way to specify this extra detail. As a result, they are

disabled for many-to-many relationships that use an intermediate model.

The only way to create this type of relationship is to create instances of the

intermediate model.

The :meth:`~django.db.models.fields.related.RelatedManager.remove` method is

disabled for similar reasons. However, the

:meth:`~django.db.models.fields.related.RelatedManager.clear` method can be

used to remove all many-to-many relationships for an instance::

    # Beatles have broken up

    >>> beatles.members.clear()

Once you have established the many-to-many relationships by creating instances

of your intermediate model, you can issue queries. Just as with normal

many-to-many relationships, you can query using the attributes of the

many-to-many-related model::

    # Find all the groups with a member whose name starts with 'Paul'

    >>> Group.objects.filter(members__name__startswith='Paul')

    [<Group: The Beatles>]

As you are using an intermediate model, you can also query on its attributes::

    # Find all the members of the Beatles that joined after 1 Jan 1961

    >>> Person.objects.filter(

    ...     group__name='The Beatles',

    ...     membership__date_joined__gt=date(1961,1,1))

    [<Person: Ringo Starr]

One-to-one relationships

~~~~~~~~~~~~~~~~~~~~~~~~

To define a one-to-one relationship, use

:class:`~django.db.models.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.