Unified some doc links to OneToOneField and ManyToManyField.

    If in doubt, leave it to its default of ``True``.

.. _ref-manytomany:

``ManyToManyField``

-------------------

:attr:`~Field.null` has no effect since there is no way to require a

relationship at the database level.

.. _ref-onetoone:

``OneToOneField``

-----------------

  <test-case-serialized-rollback>`.

* It is not advised to have apps without migrations depend on (have a

  :ref:`ForeignKey <ref-foreignkey>` or :ref:`ManyToManyField <ref-manytomany>`

  to) apps with migrations.

  :class:`~django.db.models.ForeignKey` or

  :class:`~django.db.models.ManyToManyField` to) apps with migrations.

.. _app-loading-refactor-17-release-note:

allows for any of the features offered by proxy models including default

ordering, custom managers, or custom model methods.

If you wish to store information related to ``User``, you can use a :ref:`one-to-one

relationship <ref-onetoone>` to a model containing the fields for

If you wish to store information related to ``User``, you can use a

:class:`~django.db.models.OneToOneField` to a model containing the fields for

additional information. This one-to-one model is often called a profile model,

as it might store non-auth related information about a site user. For example

you might create an Employee model::

.. highlight:: pycon

To define a many-to-many relationship, use :ref:`ref-manytomany`.

To define a many-to-many relationship, use

:class:`~django.db.models.ManyToManyField`.

In this example, an ``Article`` can be published in multiple ``Publication``

objects, and a ``Publication`` has multiple ``Article`` objects:

One-to-one relationships

========================

To define a one-to-one relationship, use :ref:`ref-onetoone`.

To define a one-to-one relationship, use

:class:`~django.db.models.OneToOneField`.

In this example, a ``Place`` optionally can be a ``Restaurant``::