Constraints reference

The classes defined in this module create database constraints. They are added in the model Meta.constraints option.

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 as models.<Foo>Constraint.

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 (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').

Validation of Constraints

In general constraints are not checked during full_clean(), and do not raise ValidationErrors. Rather you'll get a database integrity error on save(). 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 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

CheckConstraint.check

A Q object or boolean 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.

Changed in Django 3.1:

增加了对布尔 Expression 的支持。

name

CheckConstraint.name

The name of the constraint. You must always specify a unique name for the constraint.

Changed in Django 3.0:

增加了 '%(app_label)s' 和 '%(class)s' 插值。

UniqueConstraint

class UniqueConstraint(*, fields, name, condition=None, deferrable=None)

Creates a unique constraint in the database.

fields

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

UniqueConstraint.name

The name of the constraint. You must always specify a unique name for the constraint.

Changed in Django 3.0:

增加了 '%(app_label)s' 和 '%(class)s' 插值。

condition

UniqueConstraint.condition

A Q object that specifies the condition you want the constraint to enforce.

例子：

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 Index.condition.

deferrable

UniqueConstraint.deferrable
New in Django 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,
)

默认情况下，约束条件是不递延的。推迟的约束条件在事务结束前不会被强制执行。即时约束将在每条命令后立即执行。

MySQL, MariaDB, and SQLite.

Deferrable unique constraints are ignored on MySQL, MariaDB, and SQLite as neither supports them.

警告

Deferred unique constraints may lead to a performance penalty.

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