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Thanks a lot to everybody participating in developing this feature. The patch was developed by multiple people, at least Trac aliases tonnzor, jimmysong, Fandekasp and slurms. Stylistic changes added by committer.
298 lines
9.9 KiB
Plaintext
298 lines
9.9 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 exists outside of the standard :file:`models.py` (for instance,
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if the app's models are in submodules of ``myapp.models``), the model must
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define which app it is part of::
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app_label = 'myapp'
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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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``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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``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:`syncdb` and remove them as part of a :djadmin:`flush`
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management command. That is, Django *manages* the database tables'
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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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Marks this object as "orderable" with respect to the given field. This is almost
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always used with related objects to allow them to be ordered with respect to a
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parent object. For example, if an ``Answer`` relates to a ``Question`` object,
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and a question has more than one answer, and the order of answers matters, you'd
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do this::
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class Answer(models.Model):
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question = models.ForeignKey(Question)
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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:: Changing order_with_respect_to
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``order_with_respect_to`` adds an additional field/database column
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named ``_order``, so be sure to handle that as you would any other
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change to your models if you add or change ``order_with_respect_to``
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after your initial :djadmin:`syncdb`.
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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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``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 object
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that has ``admin`` set. This example specifies an extra permission,
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``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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``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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``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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``index_together``
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.. attribute:: Options.index_together
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.. versionadded:: 1.5
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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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``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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