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284 lines
9.6 KiB
Text
284 lines
9.6 KiB
Text
======================
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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 a :class:`DateField` or :class:`DateTimeField` in the model.
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This specifies the default field to use in your model :class:`Manager`'s
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:class:`~QuerySet.latest` method.
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Example::
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get_latest_by = "order_date"
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See the docs for :meth:`~django.db.models.query.QuerySet.latest` 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:`reset`
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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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.. versionchanged:: 1.4
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The Django admin honors all elements in the list/tuple; before 1.4, only
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the first one was respected.
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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 list of lists of fields 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 list 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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``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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