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Constraints reference

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

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.

name

CheckConstraint.name

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

UniqueConstraint

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

Creates a unique constraint in the database.

expressions

UniqueConstraint.expressions

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

Por exemplo:

Code
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.

name

UniqueConstraint.name

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

condition

UniqueConstraint.condition

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

Por exemplo:

Code
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:

Code
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.

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).

Por exemplo:

Code
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.

Por exemplo:

Code
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.

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