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

Constraints are checked during the model validation.

Changed in Django 4.1:

In older versions, constraints were not checked during model validation.

BaseConstraint

class BaseConstraint(name, violation_error_message=None)

Base class for all constraints. Subclasses must implement constraint_sql(), create_sql(), remove_sql() and validate() methods.

All constraints have the following parameters in common:

name

BaseConstraint.name

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

violation_error_message

New in Django 4.1.

BaseConstraint.violation_error_message

The error message used when ValidationError is raised during model validation. Defaults to "Constraint “%(name)s” is violated.".

validate()

New in Django 4.1.

BaseConstraint.validate(model, instance, exclude=None, using=DEFAULT_DB_ALIAS)

Validates that the constraint, defined on model, is respected on the instance. This will do a query on the database to ensure that the constraint is respected. If fields in the exclude list are needed to validate the constraint, the constraint is ignored.

Raise a ValidationError if the constraint is violated.

This method must be implemented by a subclass.

CheckConstraint

class CheckConstraint(**, check, name, violation_error_message=None*)

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 4.1:

The violation_error_message argument was added.

UniqueConstraint

class UniqueConstraint(\expressions, fields=(), name=None, condition=None, deferrable=None, include=None, opclasses=(), violation_error_message=None*)

Creates a unique constraint in the database.

expressions

UniqueConstraint.expressions

New in Django 4.0.

Positional argument *expressions allows creating functional unique constraints on expressions and database functions.

For example:

UniqueConstraint(Lower('name').desc(), 'category', name='unique_lower_name_category')

creates a unique constraint on the lowercased value of the name field in descending order and the category field in the default ascending order.

Functional unique constraints have the same database restrictions as Index.expressions.

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.

condition

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.

deferrable

UniqueConstraint.deferrable

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.

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.

include

UniqueConstraint.include

A list or tuple of the names of the fields to be included in the covering unique index as non-key columns. This allows index-only scans to be used for queries that select only included fields (include) and filter only by unique fields (fields).

For example:

UniqueConstraint(name='unique_booking', fields=['room', 'date'], include=['full_name'])

will allow filtering on room and date, also selecting full_name, while fetching data only from the index.

include is supported only on PostgreSQL.

Non-key columns have the same database restrictions as Index.include.

opclasses

UniqueConstraint.opclasses

The names of the PostgreSQL operator classes to use for this unique index. If you require a custom operator class, you must provide one for each field in the index.

For example:

UniqueConstraint(name='unique_username', fields=['username'], opclasses=['varchar_pattern_ops'])

creates a unique index on username using varchar_pattern_ops.

opclasses are ignored for databases besides PostgreSQL.

violation_error_message

New in Django 4.1.

UniqueConstraint.violation_error_message

The error message used when ValidationError is raised during model validation. Defaults to BaseConstraint.violation_error_message.

This message is not used for UniqueConstraints with fields and without a condition. Such UniqueConstraints show the same message as constraints defined with Field.unique or in Meta.unique_together.