mirror of https://github.com/django/django.git
156 lines
4.7 KiB
Plaintext
156 lines
4.7 KiB
Plaintext
=====================
|
|
Constraints reference
|
|
=====================
|
|
|
|
.. module:: django.db.models.constraints
|
|
|
|
.. currentmodule:: django.db.models
|
|
|
|
The classes defined in this module create database constraints. They are added
|
|
in the model :attr:`Meta.constraints <django.db.models.Options.constraints>`
|
|
option.
|
|
|
|
.. admonition:: Referencing built-in constraints
|
|
|
|
Constraints are defined in ``django.db.models.constraints``, but for
|
|
convenience they're imported into :mod:`django.db.models`. The standard
|
|
convention is to use ``from django.db import models`` and refer to the
|
|
constraints as ``models.<Foo>Constraint``.
|
|
|
|
.. admonition:: Constraints in abstract base classes
|
|
|
|
You must always specify a unique name for the constraint. As such, you
|
|
cannot normally specify a constraint on an abstract base class, since the
|
|
:attr:`Meta.constraints <django.db.models.Options.constraints>` option is
|
|
inherited by subclasses, with exactly the same values for the attributes
|
|
(including ``name``) each time. To work around name collisions, part of the
|
|
name may contain ``'%(app_label)s'`` and ``'%(class)s'``, which are
|
|
replaced, respectively, by the lowercased app label and class name of the
|
|
concrete model. For example ``CheckConstraint(check=Q(age__gte=18),
|
|
name='%(app_label)s_%(class)s_is_adult')``.
|
|
|
|
.. admonition:: Validation of Constraints
|
|
|
|
In general constraints are **not** checked during ``full_clean()``, and do
|
|
not raise ``ValidationError``\s. Rather you'll get a database integrity
|
|
error on ``save()``. ``UniqueConstraint``\s without a
|
|
:attr:`~UniqueConstraint.condition` (i.e. non-partial unique constraints)
|
|
are different in this regard, in that they leverage the existing
|
|
``validate_unique()`` logic, and thus enable two-stage validation. In
|
|
addition to ``IntegrityError`` on ``save()``, ``ValidationError`` is also
|
|
raised during model validation when the ``UniqueConstraint`` is violated.
|
|
|
|
``CheckConstraint``
|
|
===================
|
|
|
|
.. class:: CheckConstraint(*, check, name)
|
|
|
|
Creates a check constraint in the database.
|
|
|
|
``check``
|
|
---------
|
|
|
|
.. attribute:: CheckConstraint.check
|
|
|
|
A :class:`Q` object or boolean :class:`~django.db.models.Expression` that
|
|
specifies the check you want the constraint to enforce.
|
|
|
|
For example, ``CheckConstraint(check=Q(age__gte=18), name='age_gte_18')``
|
|
ensures the age field is never less than 18.
|
|
|
|
.. versionchanged:: 3.1
|
|
|
|
Support for boolean :class:`~django.db.models.Expression` was added.
|
|
|
|
``name``
|
|
--------
|
|
|
|
.. attribute:: CheckConstraint.name
|
|
|
|
The name of the constraint. You must always specify a unique name for the
|
|
constraint.
|
|
|
|
.. versionchanged:: 3.0
|
|
|
|
Interpolation of ``'%(app_label)s'`` and ``'%(class)s'`` was added.
|
|
|
|
``UniqueConstraint``
|
|
====================
|
|
|
|
.. class:: UniqueConstraint(*, fields, name, condition=None, deferrable=None)
|
|
|
|
Creates a unique constraint in the database.
|
|
|
|
``fields``
|
|
----------
|
|
|
|
.. attribute:: UniqueConstraint.fields
|
|
|
|
A list of field names that specifies the unique set of columns you want the
|
|
constraint to enforce.
|
|
|
|
For example, ``UniqueConstraint(fields=['room', 'date'],
|
|
name='unique_booking')`` ensures each room can only be booked once for each
|
|
date.
|
|
|
|
``name``
|
|
--------
|
|
|
|
.. attribute:: UniqueConstraint.name
|
|
|
|
The name of the constraint. You must always specify a unique name for the
|
|
constraint.
|
|
|
|
.. versionchanged:: 3.0
|
|
|
|
Interpolation of ``'%(app_label)s'`` and ``'%(class)s'`` was added.
|
|
|
|
``condition``
|
|
-------------
|
|
|
|
.. attribute:: UniqueConstraint.condition
|
|
|
|
A :class:`Q` object that specifies the condition you want the constraint to
|
|
enforce.
|
|
|
|
For example::
|
|
|
|
UniqueConstraint(fields=['user'], condition=Q(status='DRAFT'), name='unique_draft_user')
|
|
|
|
ensures that each user only has one draft.
|
|
|
|
These conditions have the same database restrictions as
|
|
:attr:`Index.condition`.
|
|
|
|
``deferrable``
|
|
--------------
|
|
|
|
.. attribute:: UniqueConstraint.deferrable
|
|
|
|
.. versionadded:: 3.1
|
|
|
|
Set this parameter to create a deferrable unique constraint. Accepted values
|
|
are ``Deferrable.DEFERRED`` or ``Deferrable.IMMEDIATE``. For example::
|
|
|
|
from django.db.models import Deferrable, UniqueConstraint
|
|
|
|
UniqueConstraint(
|
|
name='unique_order',
|
|
fields=['order'],
|
|
deferrable=Deferrable.DEFERRED,
|
|
)
|
|
|
|
By default constraints are not deferred. A deferred constraint will not be
|
|
enforced until the end of the transaction. An immediate constraint will be
|
|
enforced immediately after every command.
|
|
|
|
.. admonition:: MySQL, MariaDB, and SQLite.
|
|
|
|
Deferrable unique constraints are ignored on MySQL, MariaDB, and SQLite as
|
|
neither supports them.
|
|
|
|
.. warning::
|
|
|
|
Deferred unique constraints may lead to a `performance penalty
|
|
<https://www.postgresql.org/docs/current/sql-createtable.html#id-1.9.3.85.9.4>`_.
|