The classes defined in this module create database constraints. They are added
in the model
Referencing built-in constraints
Constraints are defined in
django.db.models.constraints, but for
convenience they’re imported into
django.db.models. The standard
convention is to use
from django.db import models and refer to the
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
Meta.constraints option is
inherited by subclasses, with exactly the same values for the attributes
name) each time. To work around name collisions, part of the
name may contain
'%(class)s', which are
replaced, respectively, by the lowercased app label and class name of the
concrete model. For example
Validation of Constraints
In general constraints are not checked during
full_clean(), and do
ValidationErrors. Rather you’ll get a database integrity
UniqueConstraints without a
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
ValidationError is also
raised during model validation when the
UniqueConstraint is violated.
CheckConstraint(*, check, name)¶
Creates a check constraint in the database.
ensures the age field is never less than 18.
Support for boolean
Expression was added.
UniqueConstraint(*, fields, name, condition=None, deferrable=None)¶
Creates a unique constraint in the database.
A list of field names that specifies the unique set of columns you want the constraint to enforce.
name='unique_booking') ensures each room can only be booked once for each
The name of the constraint. You must always specify a unique name for the constraint.
'%(class)s' was added.
Q object that specifies the condition you want the constraint to
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
Set this parameter to create a deferrable unique constraint. Accepted values
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.
MySQL, MariaDB, and SQLite.
Deferrable unique constraints are ignored on MySQL, MariaDB, and SQLite as neither supports them.
Deferred unique constraints may lead to a performance penalty.