docs/ref/models/options.txt
======================
Model ``Meta`` options
======================
This document explains all the possible :ref:`metadata options
<meta-options>` that you can give your model in its internal
``class Meta``.
Available ``Meta`` options
==========================
.. currentmodule:: django.db.models
``abstract``
------------
.. attribute:: Options.abstract
If ``abstract = True``, this model will be an
:ref:`abstract base class <abstract-base-classes>`.
``app_label``
-------------
.. attribute:: Options.app_label
If a model is defined outside of an application in
:setting:`INSTALLED_APPS`, it must declare which app it belongs to::
app_label = "myapp"
If you want to represent a model with the format ``app_label.object_name``
or ``app_label.model_name`` you can use ``model._meta.label``
or ``model._meta.label_lower`` respectively.
``base_manager_name``
---------------------
.. attribute:: Options.base_manager_name
The attribute name of the manager, for example, ``'objects'``, to use for
the model's :attr:`~django.db.models.Model._base_manager`.
``db_table``
------------
.. attribute:: Options.db_table
The name of the database table to use for the model::
db_table = "music_album"
.. _table-names:
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 :djadmin:`manage.py startapp <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``.
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.
.. admonition:: Use lowercase table names for MariaDB and MySQL
It is strongly advised that you use lowercase table names when you override
the table name via ``db_table``, particularly if you are using the MySQL
backend. See the :ref:`MySQL notes <mysql-notes>` for more details.
.. admonition:: Table name quoting for Oracle
In order to meet the 30-char limitation Oracle has on table names,
and match the usual conventions for Oracle databases, Django may shorten
table names and turn them all-uppercase. To prevent such transformations,
use a quoted name as the value for ``db_table``::
db_table = '"name_left_in_lowercase"'
Such quoted names can also be used with Django's other supported database
backends; except for Oracle, however, the quotes have no effect. See the
:ref:`Oracle notes <oracle-notes>` for more details.
``db_table_comment``
--------------------
.. attribute:: Options.db_table_comment
The comment on the database table to use for this model. It is useful for
documenting database tables for individuals with direct database access who may
not be looking at your Django code. For example::
class Answer(models.Model):
question = models.ForeignKey(Question, on_delete=models.CASCADE)
answer = models.TextField()
class Meta:
db_table_comment = "Question answers"
``db_tablespace``
-----------------
.. attribute:: Options.db_tablespace
The name of the :doc:`database tablespace </topics/db/tablespaces>` to use
for this model. The default is the project's :setting:`DEFAULT_TABLESPACE`
setting, if set. If the backend doesn't support tablespaces, this option is
ignored.
``default_manager_name``
------------------------
.. attribute:: Options.default_manager_name
The name of the manager to use for the model's
:attr:`~django.db.models.Model._default_manager`.
``default_related_name``
------------------------
.. attribute:: Options.default_related_name
The name that will be used by default for the relation from a related object
back to this one. The default is ``<model_name>_set``.
This option also sets :attr:`~ForeignKey.related_query_name`.
As the reverse name for a field should be unique, be careful if you intend
to subclass your model. To work around name collisions, part of the name
should contain ``'%(app_label)s'`` and ``'%(model_name)s'``, which are
replaced respectively by the name of the application the model is in,
and the name of the model, both lowercased. See the paragraph on
:ref:`related names for abstract models <abstract-related-name>`.
``get_latest_by``
-----------------
.. attribute:: Options.get_latest_by
The name of a field or a list of field names in the model, typically
:class:`DateField`, :class:`DateTimeField`, or :class:`IntegerField`. This
specifies the default field(s) to use in your model :class:`Manager`’s
:meth:`~django.db.models.query.QuerySet.latest` and
:meth:`~django.db.models.query.QuerySet.earliest` methods.
Example::
# Latest by ascending order_date.
get_latest_by = "order_date"
# Latest by priority descending, order_date ascending.
get_latest_by = ["-priority", "order_date"]
See the :meth:`~django.db.models.query.QuerySet.latest` docs for more.
``managed``
-----------
.. attribute:: Options.managed
Defaults to ``True``, meaning Django will create the appropriate database
tables in :djadmin:`migrate` or as part of migrations and remove them as
part of a :djadmin:`flush` management command. That is, Django
*manages* the database tables' lifecycles.
If ``False``, no database table creation, modification, or deletion
operations will be performed for this model. This is useful if the model
represents an existing table or a database view that has been created by
some other means. This is the *only* difference when ``managed=False``. All
other aspects of model handling are exactly the same as normal. This
includes
#. Adding an automatic primary key field to the model if you don't
declare it. To avoid confusion for later code readers, it's
recommended to specify all the columns from the database table you
are modeling when using unmanaged models.
#. If a model with ``managed=False`` contains a
:class:`~django.db.models.ManyToManyField` that points to another
unmanaged model, then the intermediate table for the many-to-many
join will also not be created. However, the intermediary table
between one managed and one unmanaged model *will* be created.
If you need to change this default behavior, create the intermediary
table as an explicit model (with ``managed`` set as needed) and use
the :attr:`ManyToManyField.through` attribute to make the relation
use your custom model.
For tests involving models with ``managed=False``, it's up to you to ensure
the correct tables are created as part of the test setup.
If you're interested in changing the Python-level behavior of a model class,
you *could* use ``managed=False`` and create a copy of an existing model.
However, there's a better approach for that situation: :ref:`proxy-models`.
``order_with_respect_to``
-------------------------
.. attribute:: Options.order_with_respect_to
Makes this object orderable with respect to the given field, usually a
``ForeignKey``. This can be used to make related objects orderable 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::
from django.db import models
class Question(models.Model):
text = models.TextField()
# ...
class Answer(models.Model):
question = models.ForeignKey(Question, on_delete=models.CASCADE)
# ...
class Meta:
order_with_respect_to = "question"
When ``order_with_respect_to`` is set, two additional methods are provided to
retrieve and to set the order of the related objects: ``get_RELATED_order()``
and ``set_RELATED_order()``, where ``RELATED`` is the lowercased model name. For
example, assuming that a ``Question`` object has multiple related ``Answer``
objects, the list returned contains the primary keys of the related ``Answer``
objects:
.. code-block:: pycon
>>> question = Question.objects.get(id=1)
>>> question.get_answer_order()
[1, 2, 3]
The order of a ``Question`` object's related ``Answer`` objects can be set by
passing in a list of ``Answer`` primary keys:
.. code-block:: pycon
>>> question.set_answer_order([3, 1, 2])
The related objects also get two methods, ``get_next_in_order()`` and
``get_previous_in_order()``, which can be used to access those objects in their
proper order. Assuming the ``Answer`` objects are ordered by ``id``:
.. code-block:: pycon
>>> answer = Answer.objects.get(id=2)
>>> answer.get_next_in_order()
<Answer: 3>
>>> answer.get_previous_in_order()
<Answer: 1>
.. admonition:: ``order_with_respect_to`` implicitly sets the ``ordering`` option
Internally, ``order_with_respect_to`` adds an additional field/database
column named ``_order`` and sets the model's :attr:`~Options.ordering`
option to this field. Consequently, ``order_with_respect_to`` and
``ordering`` cannot be used together, and the ordering added by
``order_with_respect_to`` will apply whenever you obtain a list of objects
of this model.
.. admonition:: Changing ``order_with_respect_to``
Because ``order_with_respect_to`` adds a new database column, be sure to
make and apply the appropriate migrations if you add or change
``order_with_respect_to`` after your initial :djadmin:`migrate`.
``ordering``
------------
.. attribute:: Options.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 and/or query expressions. 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"]
You can also use :doc:`query expressions </ref/models/expressions>`. To
order by ``author`` ascending and make null values sort last, use this::
from django.db.models import F
ordering = [F("author").asc(nulls_last=True)]
.. warning::
Ordering is not a free operation. Each field you add to the ordering
incurs a cost to your database. Each foreign key you add will
implicitly include all of its default orderings as well.
If a query doesn't have an ordering specified, results are returned from
the database in an unspecified order. A particular ordering is guaranteed
only when ordering by a set of fields that uniquely identify each object in
the results. For example, if a ``name`` field isn't unique, ordering by it
won't guarantee objects with the same name always appear in the same order.
``permissions``
---------------
.. attribute:: Options.permissions
Extra permissions to enter into the permissions table when creating this object.
Add, change, delete, and view permissions are automatically created for each
model. 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)``.
``default_permissions``
-----------------------
.. attribute:: Options.default_permissions
Defaults to ``('add', 'change', 'delete', 'view')``. You may customize this
list, for example, by setting this to an empty list if your app doesn't
require any of the default permissions. It must be specified on the model
before the model is created by :djadmin:`migrate` in order to prevent any
omitted permissions from being created.
``proxy``
---------
.. attribute:: Options.proxy
If ``proxy = True``, a model which subclasses another model will be treated as
a :ref:`proxy model <proxy-models>`.
``required_db_features``
------------------------
.. attribute:: Options.required_db_features
List of database features that the current connection should have so that
the model is considered during the migration phase. For example, if you set
this list to ``['gis_enabled']``, the model will only be synchronized on
GIS-enabled databases. It's also useful to skip some models when testing
with several database backends. Avoid relations between models that may or
may not be created as the ORM doesn't handle this.
``required_db_vendor``
----------------------
.. attribute:: Options.required_db_vendor
Name of a supported database vendor that this model is specific to. Current
built-in vendor names are: ``sqlite``, ``postgresql``, ``mysql``,
``oracle``. If this attribute is not empty and the current connection vendor
doesn't match it, the model will not be synchronized.
``select_on_save``
------------------
.. attribute:: Options.select_on_save
Determines if Django will use the pre-1.6
:meth:`django.db.models.Model.save()` algorithm. The old algorithm
uses ``SELECT`` to determine if there is an existing row to be updated.
The new algorithm tries an ``UPDATE`` directly. In some rare cases the
``UPDATE`` of an existing row isn't visible to Django. An example is the
PostgreSQL ``ON UPDATE`` trigger which returns ``NULL``. In such cases the
new algorithm will end up doing an ``INSERT`` even when a row exists in
the database.
Usually there is no need to set this attribute. The default is
``False``.
See :meth:`django.db.models.Model.save()` for more about the old and
new saving algorithm.
``indexes``
-----------
.. attribute:: Options.indexes
A list of :doc:`indexes </ref/models/indexes>` that you want to define on
the model::
from django.db import models
class Customer(models.Model):
first_name = models.CharField(max_length=100)
last_name = models.CharField(max_length=100)
class Meta:
indexes = [
models.Index(fields=["last_name", "first_name"]),
models.Index(fields=["first_name"], name="first_name_idx"),
]
``unique_together``
-------------------
.. attribute:: Options.unique_together
.. admonition:: Use :class:`.UniqueConstraint` with the :attr:`~Options.constraints` option instead.
:class:`.UniqueConstraint` provides more functionality than
``unique_together``. ``unique_together`` may be deprecated in the
future.
Sets of field names that, taken together, must be unique::
unique_together = [["driver", "restaurant"]]
This is a list of lists 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).
For convenience, ``unique_together`` can be a single list when dealing with
a single set of fields::
unique_together = ["driver", "restaurant"]
A :class:`~django.db.models.ManyToManyField` cannot be included in
``unique_together``. (It's not clear what that would even mean!) If you
need to validate uniqueness related to a
:class:`~django.db.models.ManyToManyField`, try using a signal or
an explicit :attr:`through <ManyToManyField.through>` model.
The ``ValidationError`` raised during model validation when the constraint
is violated has the ``unique_together`` error code.
``constraints``
---------------
.. attribute:: Options.constraints
A list of :doc:`constraints </ref/models/constraints>` that you want to
define on the model::
from django.db import models
class Customer(models.Model):
age = models.IntegerField()
class Meta:
constraints = [
models.CheckConstraint(condition=models.Q(age__gte=18), name="age_gte_18"),
]
``verbose_name``
----------------
.. attribute:: Options.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``
-----------------------
.. attribute:: Options.verbose_name_plural
The plural name for the object::
verbose_name_plural = "stories"
If this isn't given, Django will use :attr:`~Options.verbose_name` + ``"s"``.
Read-only ``Meta`` attributes
=============================
``label``
---------
.. attribute:: Options.label
Representation of the object, returns ``app_label.object_name``, e.g.
``'polls.Question'``.
``label_lower``
---------------
.. attribute:: Options.label_lower
Representation of the model, returns ``app_label.model_name``, e.g.
``'polls.question'``.