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.
Validation of Constraints with JSONField
Constraints containing JSONField may not raise
validation errors as key, index, and path transforms have many
database-specific caveats. This may be fully supported later.
You should always check that there are no log messages, in the
django.db.models logger, like “Got a database error calling check() on
…” to confirm it’s validated properly.
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()andvalidate()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¶
-
BaseConstraint.violation_error_message¶
The error message used when ValidationError is raised during
model validation. Defaults to
"Constraint “%(name)s” is violated.".
validate()¶
-
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.
Oracle
Checks with nullable fields on Oracle must include a condition allowing for
NULL values in order for validate()
to behave the same as check constraints validation. For example, if age
is a nullable field:
CheckConstraint(check=Q(age__gte=18) | Q(age__isnull=True), name='age_gte_18')
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¶
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¶
-
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.
