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468 lines
16 KiB
Plaintext
======================
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Model ``Meta`` options
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======================
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This document explains all the possible :ref:`metadata options
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<meta-options>` that you can give your model in its internal
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``class Meta``.
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Available ``Meta`` options
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==========================
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.. currentmodule:: django.db.models
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``abstract``
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------------
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.. attribute:: Options.abstract
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If ``abstract = True``, this model will be an
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:ref:`abstract base class <abstract-base-classes>`.
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``app_label``
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-------------
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.. attribute:: Options.app_label
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If a model is defined outside of an application in
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:setting:`INSTALLED_APPS`, it must declare which app it belongs to::
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app_label = 'myapp'
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.. versionadded:: 1.9
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If you want to represent a model with the format ``app_label.object_name``
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or ``app_label.model_name`` you can use ``model._meta.label``
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or ``model._meta.label_lower`` respectively.
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``db_table``
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------------
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.. attribute:: Options.db_table
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The name of the database table to use for the model::
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db_table = 'music_album'
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.. _table-names:
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Table names
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~~~~~~~~~~~
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To save you time, Django automatically derives the name of the database table
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from the name of your model class and the app that contains it. A model's
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database table name is constructed by joining the model's "app label" -- the
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name you used in :djadmin:`manage.py startapp <startapp>` -- to the model's
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class name, with an underscore between them.
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For example, if you have an app ``bookstore`` (as created by
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``manage.py startapp bookstore``), a model defined as ``class Book`` will have
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a database table named ``bookstore_book``.
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To override the database table name, use the ``db_table`` parameter in
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``class Meta``.
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If your database table name is an SQL reserved word, or contains characters that
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aren't allowed in Python variable names -- notably, the hyphen -- that's OK.
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Django quotes column and table names behind the scenes.
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.. admonition:: Use lowercase table names for MySQL
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It is strongly advised that you use lowercase table names when you override
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the table name via ``db_table``, particularly if you are using the MySQL
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backend. See the :ref:`MySQL notes <mysql-notes>` for more details.
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.. admonition:: Table name quoting for Oracle
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In order to meet the 30-char limitation Oracle has on table names,
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and match the usual conventions for Oracle databases, Django may shorten
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table names and turn them all-uppercase. To prevent such transformations,
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use a quoted name as the value for ``db_table``::
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db_table = '"name_left_in_lowercase"'
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Such quoted names can also be used with Django's other supported database
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backends; except for Oracle, however, the quotes have no effect. See the
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:ref:`Oracle notes <oracle-notes>` for more details.
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``db_tablespace``
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-----------------
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.. attribute:: Options.db_tablespace
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The name of the :doc:`database tablespace </topics/db/tablespaces>` to use
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for this model. The default is the project's :setting:`DEFAULT_TABLESPACE`
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setting, if set. If the backend doesn't support tablespaces, this option is
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ignored.
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``default_related_name``
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------------------------
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.. attribute:: Options.default_related_name
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The name that will be used by default for the relation from a related object
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back to this one. The default is ``<model_name>_set``.
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This option also sets :attr:`~ForeignKey.related_query_name`.
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As the reverse name for a field should be unique, be careful if you intend
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to subclass your model. To work around name collisions, part of the name
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should contain ``'%(app_label)s'`` and ``'%(model_name)s'``, which are
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replaced respectively by the name of the application the model is in,
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and the name of the model, both lowercased. See the paragraph on
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:ref:`related names for abstract models <abstract-related-name>`.
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.. deprecated:: 1.10
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This attribute now affects ``related_query_name``. The old query lookup
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name is deprecated::
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from django.db import models
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class Foo(models.Model):
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pass
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class Bar(models.Model):
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foo = models.ForeignKey(Foo)
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class Meta:
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default_related_name = 'bars'
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::
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>>> bar = Bar.objects.get(pk=1)
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>>> # Using model name "bar" as lookup string is deprecated.
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>>> Foo.objects.get(bar=bar)
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>>> # You should use default_related_name "bars".
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>>> Foo.objects.get(bars=bar)
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``get_latest_by``
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-----------------
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.. attribute:: Options.get_latest_by
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The name of an orderable field in the model, typically a :class:`DateField`,
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:class:`DateTimeField`, or :class:`IntegerField`. This specifies the default
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field to use in your model :class:`Manager`’s
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:meth:`~django.db.models.query.QuerySet.latest` and
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:meth:`~django.db.models.query.QuerySet.earliest` methods.
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Example::
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get_latest_by = "order_date"
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See the :meth:`~django.db.models.query.QuerySet.latest` docs for more.
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``managed``
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-----------
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.. attribute:: Options.managed
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Defaults to ``True``, meaning Django will create the appropriate database
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tables in :djadmin:`migrate` or as part of migrations and remove them as
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part of a :djadmin:`flush` management command. That is, Django
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*manages* the database tables' lifecycles.
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If ``False``, no database table creation or deletion operations will be
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performed for this model. This is useful if the model represents an existing
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table or a database view that has been created by some other means. This is
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the *only* difference when ``managed=False``. All other aspects of
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model handling are exactly the same as normal. This includes
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1. Adding an automatic primary key field to the model if you don't
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declare it. To avoid confusion for later code readers, it's
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recommended to specify all the columns from the database table you
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are modeling when using unmanaged models.
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2. If a model with ``managed=False`` contains a
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:class:`~django.db.models.ManyToManyField` that points to another
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unmanaged model, then the intermediate table for the many-to-many
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join will also not be created. However, the intermediary table
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between one managed and one unmanaged model *will* be created.
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If you need to change this default behavior, create the intermediary
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table as an explicit model (with ``managed`` set as needed) and use
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the :attr:`ManyToManyField.through` attribute to make the relation
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use your custom model.
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For tests involving models with ``managed=False``, it's up to you to ensure
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the correct tables are created as part of the test setup.
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If you're interested in changing the Python-level behavior of a model class,
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you *could* use ``managed=False`` and create a copy of an existing model.
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However, there's a better approach for that situation: :ref:`proxy-models`.
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``order_with_respect_to``
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-------------------------
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.. attribute:: Options.order_with_respect_to
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Makes this object orderable with respect to the given field, usually a
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``ForeignKey``. This can be used to make related objects orderable with
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respect to a parent object. For example, if an ``Answer`` relates to a
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``Question`` object, and a question has more than one answer, and the order
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of answers matters, you'd do this::
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from django.db import models
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class Question(models.Model):
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text = models.TextField()
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# ...
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class Answer(models.Model):
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question = models.ForeignKey(Question, on_delete=models.CASCADE)
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# ...
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class Meta:
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order_with_respect_to = 'question'
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When ``order_with_respect_to`` is set, two additional methods are provided to
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retrieve and to set the order of the related objects: ``get_RELATED_order()``
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and ``set_RELATED_order()``, where ``RELATED`` is the lowercased model name. For
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example, assuming that a ``Question`` object has multiple related ``Answer``
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objects, the list returned contains the primary keys of the related ``Answer``
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objects::
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>>> question = Question.objects.get(id=1)
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>>> question.get_answer_order()
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[1, 2, 3]
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The order of a ``Question`` object's related ``Answer`` objects can be set by
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passing in a list of ``Answer`` primary keys::
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>>> question.set_answer_order([3, 1, 2])
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The related objects also get two methods, ``get_next_in_order()`` and
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``get_previous_in_order()``, which can be used to access those objects in their
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proper order. Assuming the ``Answer`` objects are ordered by ``id``::
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>>> answer = Answer.objects.get(id=2)
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>>> answer.get_next_in_order()
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<Answer: 3>
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>>> answer.get_previous_in_order()
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<Answer: 1>
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.. admonition:: ``order_with_respect_to`` implicitly sets the ``ordering`` option
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Internally, ``order_with_respect_to`` adds an additional field/database
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column named ``_order`` and sets the model's :attr:`~Options.ordering`
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option to this field. Consequently, ``order_with_respect_to`` and
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``ordering`` cannot be used together, and the ordering added by
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``order_with_respect_to`` will apply whenever you obtain a list of objects
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of this model.
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.. admonition:: Changing ``order_with_respect_to``
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Because ``order_with_respect_to`` adds a new database column, be sure to
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make and apply the appropriate migrations if you add or change
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``order_with_respect_to`` after your initial :djadmin:`migrate`.
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``ordering``
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------------
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.. attribute:: Options.ordering
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The default ordering for the object, for use when obtaining lists of objects::
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ordering = ['-order_date']
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This is a tuple or list of strings. Each string is a field name with an optional
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"-" prefix, which indicates descending order. Fields without a leading "-" will
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be ordered ascending. Use the string "?" to order randomly.
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For example, to order by a ``pub_date`` field ascending, use this::
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ordering = ['pub_date']
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To order by ``pub_date`` descending, use this::
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ordering = ['-pub_date']
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To order by ``pub_date`` descending, then by ``author`` ascending, use this::
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ordering = ['-pub_date', 'author']
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.. warning::
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Ordering is not a free operation. Each field you add to the ordering
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incurs a cost to your database. Each foreign key you add will
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implicitly include all of its default orderings as well.
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If a query doesn't have an ordering specified, results are returned from
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the database in an unspecified order. A particular ordering is guaranteed
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only when ordering by a set of fields that uniquely identify each object in
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the results. For example, if a ``name`` field isn't unique, ordering by it
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won't guarantee objects with the same name always appear in the same order.
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``permissions``
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---------------
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.. attribute:: Options.permissions
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Extra permissions to enter into the permissions table when creating this object.
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Add, delete and change permissions are automatically created for each
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model. This example specifies an extra permission, ``can_deliver_pizzas``::
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permissions = (("can_deliver_pizzas", "Can deliver pizzas"),)
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This is a list or tuple of 2-tuples in the format ``(permission_code,
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human_readable_permission_name)``.
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``default_permissions``
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------------------------------
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.. attribute:: Options.default_permissions
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Defaults to ``('add', 'change', 'delete')``. You may customize this list,
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for example, by setting this to an empty list if your app doesn't require
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any of the default permissions. It must be specified on the model before
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the model is created by :djadmin:`migrate` in order to prevent any omitted
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permissions from being created.
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``proxy``
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---------
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.. attribute:: Options.proxy
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If ``proxy = True``, a model which subclasses another model will be treated as
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a :ref:`proxy model <proxy-models>`.
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``required_db_features``
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------------------------
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.. attribute:: Options.required_db_features
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.. versionadded:: 1.9
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List of database features that the current connection should have so that
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the model is considered during the migration phase. For example, if you set
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this list to ``['gis_enabled']``, the model will only be synchronized on
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GIS-enabled databases. It's also useful to skip some models when testing
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with several database backends. Avoid relations between models that may or
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may not be created as the ORM doesn't handle this.
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``required_db_vendor``
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----------------------
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.. attribute:: Options.required_db_vendor
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.. versionadded:: 1.9
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Name of a supported database vendor that this model is specific to. Current
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built-in vendor names are: ``sqlite``, ``postgresql``, ``mysql``,
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``oracle``. If this attribute is not empty and the current connection vendor
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doesn't match it, the model will not be synchronized.
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``select_on_save``
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------------------
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.. attribute:: Options.select_on_save
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Determines if Django will use the pre-1.6
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:meth:`django.db.models.Model.save()` algorithm. The old algorithm
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uses ``SELECT`` to determine if there is an existing row to be updated.
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The new algorithm tries an ``UPDATE`` directly. In some rare cases the
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``UPDATE`` of an existing row isn't visible to Django. An example is the
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PostgreSQL ``ON UPDATE`` trigger which returns ``NULL``. In such cases the
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new algorithm will end up doing an ``INSERT`` even when a row exists in
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the database.
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Usually there is no need to set this attribute. The default is
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``False``.
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See :meth:`django.db.models.Model.save()` for more about the old and
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new saving algorithm.
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``unique_together``
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-------------------
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.. attribute:: Options.unique_together
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Sets of field names that, taken together, must be unique::
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unique_together = (("driver", "restaurant"),)
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This is a tuple of tuples that must be unique when considered together.
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It's used in the Django admin and is enforced at the database level (i.e., the
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appropriate ``UNIQUE`` statements are included in the ``CREATE TABLE``
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statement).
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For convenience, unique_together can be a single tuple when dealing with a single
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set of fields::
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unique_together = ("driver", "restaurant")
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A :class:`~django.db.models.ManyToManyField` cannot be included in
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unique_together. (It's not clear what that would even mean!) If you
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need to validate uniqueness related to a
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:class:`~django.db.models.ManyToManyField`, try using a signal or
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an explicit :attr:`through <ManyToManyField.through>` model.
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The ``ValidationError`` raised during model validation when the constraint
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is violated has the ``unique_together`` error code.
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``index_together``
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------------------
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.. attribute:: Options.index_together
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Sets of field names that, taken together, are indexed::
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index_together = [
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["pub_date", "deadline"],
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]
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This list of fields will be indexed together (i.e. the appropriate
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``CREATE INDEX`` statement will be issued.)
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For convenience, ``index_together`` can be a single list when dealing with a single
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set of fields::
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index_together = ["pub_date", "deadline"]
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``verbose_name``
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----------------
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.. attribute:: Options.verbose_name
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A human-readable name for the object, singular::
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verbose_name = "pizza"
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If this isn't given, Django will use a munged version of the class name:
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``CamelCase`` becomes ``camel case``.
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``verbose_name_plural``
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-----------------------
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.. attribute:: Options.verbose_name_plural
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The plural name for the object::
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verbose_name_plural = "stories"
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If this isn't given, Django will use :attr:`~Options.verbose_name` + ``"s"``.
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Read-only ``Meta`` attributes
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=============================
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``label``
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---------
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.. attribute:: Options.label
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.. versionadded:: 1.9
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Representation of the object, returns ``app_label.object_name``, e.g.
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``'polls.Question'``.
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``label_lower``
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---------------
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.. attribute:: Options.label_lower
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.. versionadded:: 1.9
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Representation of the model, returns ``app_label.model_name``, e.g.
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``'polls.question'``.
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