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 fromdjango.dbimportmodels 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.
BaseConstraint¶
- classBaseConstraint(*name, violation_error_code=None, violation_error_message=None)[source]¶
Base class for all constraints. Subclasses must implement
constraint_sql(),create_sql(),remove_sql()andvalidate()methods.Deprecated since version 5.0: Support for passing positional arguments is deprecated.
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_code¶
- BaseConstraint.violation_error_code¶
The error code used when ValidationError is raised during model validation. Defaults to None.
violation_error_message¶
- BaseConstraint.violation_error_message¶
The error message used when ValidationError is raised during model validation. Defaults to "Constraint“%(name)s”isviolated.".
validate()¶
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¶
- classCheckConstraint(*, check, name, violation_error_code=None, violation_error_message=None)[source]¶
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.
Expression order
Q argument order is not necessarily preserved, however the order of Q expressions themselves are preserved. This may be important for databases that preserve check constraint expression order for performance reasons. For example, use the following format if order matters:
CheckConstraint(check=Q(age__gte=18)&Q(expensive_check=condition),name="age_gte_18_and_others",)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")UniqueConstraint¶
- classUniqueConstraint(*expressions, fields=(), name=None, condition=None, deferrable=None, include=None, opclasses=(), nulls_distinct=None, violation_error_code=None, violation_error_message=None)[source]¶
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:
fromdjango.db.modelsimportDeferrable,UniqueConstraintUniqueConstraint(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.
Unique constraints with non-key columns are ignored for databases besides 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.
nulls_distinct¶
- UniqueConstraint.nulls_distinct¶
Whether rows containing NULL values covered by the unique constraint should be considered distinct from each other. The default value is None which uses the database default which is True on most backends.
For example:
UniqueConstraint(name="ordering",fields=["ordering"],nulls_distinct=False)creates a unique constraint that only allows one row to store a NULL value in the ordering column.
Unique constraints with nulls_distinct are ignored for databases besides PostgreSQL 15+.
violation_error_code¶
- UniqueConstraint.violation_error_code¶
The error code used when ValidationError is raised during model validation. Defaults to None.
This code is not used for UniqueConstraints with fields and without a condition. Such UniqueConstraints have the same error code as constraints defined with Field.unique or in Meta.unique_together.
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.
