root/django/branches/0.90-bugfixes/docs/model-api.txt

===============
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.core.meta.Model``.
    * Each attribute of the model represents a database field.
    * Model metadata (non-field information) goes in an inner class named ``META``.

A companion to this document is the `official repository of model examples`_.

.. _`official repository of model examples`: http://www.djangoproject.com/documentation/models/

Field objects
=============

The most important part of a model is the list of database fields it defines.
Fields are defined by class attributes. Each class attribute in a model, aside
from the optional inner ``class META``, should be an instance of a
``meta.Field`` subclass.

In this example, there are two fields, ``first_name`` and ``last_name`` ::

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

Django will use ``first_name`` and ``last_name`` as the database column names.

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

In this example, the human-readable name is ``"Person's first name"``::

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

In this example, the human-readable name is ``"first name"``::

    first_name = meta.CharField(maxlength=30)

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

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

General 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``
    A list 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.

``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.

``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 = meta.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

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 (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
    ``primary_key`` in ``General field options`` above.)

``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.

        ``auto_now_add``        Automatically set the field to now when the object
                                is first created. Useful for creation of
                                timestamps.
        ======================  ===================================================

    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::

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

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

        meta.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.

    Implies ``maxlength=50`` and ``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::

        meta.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/

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

Clearly, the power of relational databases lies in relating tables to each
other. Django offers ways to define the 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 ``Place`` model is in a ``City`` -- that is, a ``City``
contains multiple places but each ``Place`` is only in one ``City`` -- here's
how you'd represent that::

    class City(meta.Model):
        # ...

    class Place(meta.Model):
        # ...
        city = meta.ForeignKey(City)

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

The name of a ``ForeignKey`` (``city`` in the example above) generally should
be the name of the model, singular. Behind the scenes, Django appends "_id" to
the field name to create its database column name. But your code should never
have to deal with the database column name, unless you write custom SQL.

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 ``meta.TABULAR`` or ``meta.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 ``meta.LazyDate`` to limit choices of objects
                             by date. For example::

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

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

                             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 just controls the
                             interface.

    ``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.  For example, when if
                             ``Topping`` has this field::

                                    meta.ForeignKey(Pizza)

                             the ``related_name`` will be "topping" (taken from
                             the class name), which will in turn give ``Pizza``
                             the methods ``get_topping_list()`` and
                             ``get_topping_count()``.

                             If you instead were to use::

                                    meta.ForeignKey(Pizza, related_name="munchie")

                             then the methods would be called
                             ``get_munchie_list()``, ``get_munchie_count()``,
                             etc.

                             This is only really useful when you have a single
                             object that relates to the same object more than
                             once.  For example, if a ``Story`` object has both
                             ``primary_category`` and ``secondary_category``
                             fields, to make sure that the ``Category`` objects
                             have the correct methods, you'd use fields like::

                                    meta.ForeignKey(Category, related_name="primary_story")
                                    meta.ForeignKey(Category, related_name="secondary_story")

                             ...which would give the ``Category`` objects
                             methods named ``get_primary_story_list()`` and
                             ``get_secondary_story_list()``.

    ``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/

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

To define a many-to-one 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(meta.Model):
        # ...

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

The name of a ``ManyToManyField`` (``toppings`` in the example above) generally
should be the name of the model, plural.

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. 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 of ``related_name`` in
                             ``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
                             ``meta.HORIZONTAL`` or ``meta.VERTICAL`` (i.e.
                             should the interface be stacked horizontally or
                             vertically).

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

    ``singular``             The singular name of the field.  Use to name the ``get_*``
                             methods: in the example above, Django gives the ``Pizza``
                             objects a ``get_topping_list()`` method, where ``topping``
                             is the default ``singular`` value derived from the lowercase
                             version of the class being linked to.  Use the singular
                             parameter to change this, which is if you want one model to
                             have multiple ``ManyToMany`` relationships to another model.
    =======================  ============================================================

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

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).

This ``OneToOneField`` will actually replace the primary key ``id`` field
(since one-to-one relations share the same primary key), and has a few
differences in the admin interface:

    * No ``Place`` selection interface is displayed on ``Restaurant`` pages.
      There will be one (and only one) ``Restaurant`` for each ``Place``.

    * On the ``Restaurant`` change list, every ``Place`` -- whether it has an
      associated ``Restaurant`` or not -- will be displayed. Adding a
      ``Restaurant`` to a ``Place`` just means filling out the required
      ``Restaurant`` fields.

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(meta.Model):
        bar = meta.CharField(maxlength=30)
        # ...
        class META:
            admin = meta.Admin()
            # ...

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

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

``admin``
    A ``meta.Admin`` object; see `Admin options`_. If this field is given, the
    object will have an admin interface. If it isn't given, the object won't
    have one.

``db_table``
    The name of the database table to use for the module::

        db_table = "pizza_orders"

    If this isn't given, Django will use ``app_label + '_' + module_name``.

    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.

``exceptions``
    Names of extra exception subclasses to include in the generated module.
    These exceptions are available from instance methods and from module-level
    methods::

        exceptions = ("DisgustingToppingsException", "BurntCrust")

``get_latest_by``
    The name of a ``DateField`` or ``DateTimeField``. If given, the module will
    have a ``get_latest()`` function that fetches the "latest" object according
    to that field::

        get_latest_by = "order_date"

    See `Getting the "latest" object`_ for a full example.

    .. _Getting the "latest" object: http://www.djangoproject.com/documentation/models/get_latest/

``module_constants``
    A dictionary of names/values to use as extra module-level constants::

        module_constants = {
            'MEAT_TYPE_PEPPERONI' : 1,
            'MEAT_TYPE_SAUSAGE' : 2,
        }

``module_name``
    The name of the module::

        module_name = "pizza_orders"

    If this isn't given, Django will use a lowercased version of the class
    name, plus ``"s"``. This "poor man's pluralization" is intentional: Any
    other level of magic pluralization would get confusing.

``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 a ``PizzaToppping`` relates to
    a ``Pizza`` object, you might use::

        order_with_respect_to = 'pizza'

    to allow the toppings to be ordered with respect to the associated pizza.

``ordering``
    The default ordering for the object, for use by ``get_list`` and the admin::

        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.

    See `Specifying ordering`_ for a full example.

    .. _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"``.

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

The ``admin`` field in the model tells Django how to construct the admin
interface for the object.  The field is an instance of the ``meta.Admin``
object, which takes the following parameters. All are optional.

``date_hierarchy``
    To allow filtering of objects in the admin by date, set ``date_hierarchy``
    to the name of the field to filter by::

        date_hierarchy = 'order_date'

``fields``
    A list of fieldsets to display on the admin page.  Each fieldset is a 2-tuple:
    ``(name, field_options)``.  The ``name`` is a string to name the field set,
    and ``field_options`` is a dictionary of information about the fields to be
    displayed in that fieldset. This dictionary has the following keys:

        ``fields``
            A tuple of field names to display in this fieldset. To display
            multiple fields on the same line, wrap those fields in their
            own tuple.

            This key is required in the dictionary.

        ``classes``
            Extra CSS classes to apply to the fieldset. This is a simple
            string. You can apply multiple classes by separating them with
            spaces.

            Two useful classes defined by the default 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.

    For example (taken from the ``django.contrib.flatpages`` model)::

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

    results in an admin that looks like:

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

    If ``fields`` isn't given but a model does define ``admin`` as a
    ``meta.Admin`` object, 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.

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

``list_display``
    List of fields to display on the list page in the admin.

    There are a few special cases that do other things besides displaying the
    contents of the given fields:

        * If the field given is a ``ForeignKey``, the ``repr()`` of the related
          object will be displayed.

        * ``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``, a "on" or "off" icon will
          be displayed instead of ``True`` or ``False``.

        * If the field name is a method of the model, it'll be called, and the
          output will be displayed. This method should have a
          ``short_description`` function attribute, for use as the header for
          the field.

        * Use the string ``"__repr__"`` to output the representation of the
          object, according to your model's ``__repr__()`` function. If you
          don't define ``list_display``, Django will use the ``__repr__`` by
          default.

    See the example below.

``list_filter``
    List of fields to filter by. Each field should either be a ``BooleanField``
    or else a field with a ``ManyToOne`` relation.

    Here's an example of how ``list_display`` and ``list_filter`` work (taken
    from the ``auth.user`` model)::

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

    This results in a admin that looks like:

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

    (This example also has ``search_fields`` defined; see below).

``list_select_related``
    Either ``True`` or ``False``. Default is ``False``. If ``True``, the admin
    change list page will use the ``select_related`` database-API parameter in
    its query that retrieves the list of items.

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

``ordering``
    A list or tuple (see the `META options`_, above) that gives a
    different ordering for the admin change list. If this isn't given, the
    model's default ordering will be used.

``save_as``
    Enables a "save as" feature on object pages.  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.

``save_on_top``
    If this option is ``True``, object pages will have the save buttons across
    the top as well as at the bottom of the page.

``search_fields``
    A list of field names to provide a text search for. These fields should,
    obviously, be some kind of text field, such as ``CharField`` or
    ``TextField``.

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

There are a number of methods you can define on model objects to control the
object's behavior.  First, any methods you define will be available as methods
of object instances. For example::

    class Pizza(meta.Model):
        # ...

        def is_disgusting(self):
            return "anchovies" in [topping.name for topping in self.get_topping_list()]

Now, every ``Pizza`` object will have a ``is_disgusting()`` method.

Note that the scope of custom methods is modified to be the same as the module
scope. These methods do NOT have access to globals within your model's module.

See `Giving models custom methods`_ for a full example.

.. _Giving models custom methods: http://www.djangoproject.com/documentation/models/custom_methods/

A few object methods have special meaning:

``__repr__``
    Django uses ``repr(obj)`` in a number of places, most notably 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 ``__repr__``.

    Although defining ``__repr__()`` isn't required, it's strongly encouraged.

    See `Adding repr`_ for a full example.

    .. _Adding repr: http://www.djangoproject.com/documentation/models/repr/

``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 "/pizzas/%i/" % self.id

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

    It's good practice to use ``get_absolute_url()`` in templates, instead of
    hard-coding your objects' URLs.

``_pre_save``
    This method is called just before an object is saved to the database. For
    example, you can use it to calculate aggregate values from other fields
    before the object is saved.

    See `Adding hooks before/after saving and deleting`_ for a full example.

    .. _Adding hooks before/after saving and deleting: http://www.djangoproject.com/documentation/models/save_delete_hooks/

``_post_save``
    This method is called just after the object is saved to the database. This
    could be used to update other tables, update cached information, etc.

``_pre_delete``
    Like ``_pre_save``, but for deletion.

``_post_delete``
    Like ``_post_save``, but for deletion.

Module-level methods
--------------------

Since each data class effectively turns into a "magic" Python module under
``django.models``, there are times you'll want to write methods that live in
that module.  Any model method that begins with "_module_" is turned into a
module-level function::

    class Pizza(meta.Model):
        # ...

        def _module_get_pizzas_to_deliver():
            return get_list(delivered__exact=False)

This will make the top-level ``pizzas`` module have a ``get_pizzas_to_deliver()``
method::

    >>> from django.models.pizza_hut import pizzas
    >>> pizzas.get_pizzas_to_deliver()
    [ ... ]

Note that the scope of these methods is modified to be the same as the module
scope. These methods do NOT have access