Django ref models constraints


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 ValidationError s. Rather you’ll get a database integrity error on save() . UniqueConstraint s 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.


class CheckConstraint ( * , check , name )

Creates a check constraint in the database.


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 version 3.1: Support for boolean Expression was added.


CheckConstraint. name

The name of the constraint.

Changed in version 3.0: Interpolation of '%(app_label)s' and '%(class)s' was added.


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

Creates a unique constraint in the database.


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.


UniqueConstraint. name

The name of the constraint.

Changed in version 3.0: Interpolation of '%(app_label)s' and '%(class)s' was added.


UniqueConstraint. condition

A 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 Index.condition .