The classes defined in this module create database constraints. They are added\nin the model Meta.constraints\noption.
BaseConstraintLink to this heading#Base class for all constraints. Subclasses must implement\nconstraint_sql(), create_sql(), remove_sql() and\nvalidate() methods.
All constraints have the following parameters in common:
\nnameLink to this heading#The name of the constraint. You must always specify a unique name for the\nconstraint.
\nviolation_error_codeLink to this heading#The error code used when ValidationError is raised during\nmodel validation. Defaults to None.
violation_error_messageLink to this heading#The error message used when ValidationError is raised during\nmodel validation. Defaults to\n"Constraint ā%(name)sā is violated.".
validate()Link to this heading#Validates that the constraint, defined on model, is respected on the\ninstance. This will do a query on the database to ensure that the\nconstraint is respected. If fields in the exclude list are needed to\nvalidate the constraint, the constraint is ignored.
Raise a ValidationError if the constraint is violated.
This method must be implemented by a subclass.
\nCheckConstraintLink to this heading#Creates a check constraint in the database.
\nconditionLink to this heading#A Q object or boolean Expression that\nspecifies the conditional check you want the constraint to enforce.
For example:
\nCheckConstraint(condition=Q(age__gte=18), name="age_gte_18")\nensures the age field is never less than 18.
\n\n\nUniqueConstraintLink to this heading#Creates a uniqueness guarantee in the database, enforced by either a\nunique constraint or a unique index depending on the options used.
\nexpressionsLink to this heading#Positional argument *expressions allows creating functional unique\nconstraints on expressions and database functions.
For example:
\nUniqueConstraint(Lower("name").desc(), "category", name="unique_lower_name_category")\ncreates a unique constraint on the lowercased value of the name field in\ndescending order and the category field in the default ascending order.
Functional unique constraints have the same database restrictions as\nIndex.expressions.
fieldsLink to this heading#A list of field names that specifies the unique set of columns you want the\nconstraint to enforce.
\nFor example:
\nUniqueConstraint(fields=["room", "date"], name="unique_booking")\nensures each room can only be booked once for each date.
\nconditionLink to this heading#A Q object that specifies the condition you want the constraint to\nenforce.
For example:
\nUniqueConstraint(fields=["user"], condition=Q(status="DRAFT"), name="unique_draft_user")\nensures that each user only has one draft.
\nThese conditions have the same database restrictions as\nIndex.condition.
deferrableLink to this heading#Set this parameter to create a deferrable unique constraint. Accepted values\nare Deferrable.DEFERRED or Deferrable.IMMEDIATE. For example:
from django.db.models import Deferrable, UniqueConstraint\n\nUniqueConstraint(\n name="unique_order",\n fields=["order"],\n deferrable=Deferrable.DEFERRED,\n)\nBy default constraints are not deferred. A deferred constraint will not be\nenforced until the end of the transaction. An immediate constraint will be\nenforced immediately after every command.
\nUnique constraints with condition,\ninclude, opclasses, or\nexpressions may be implemented as unique indexes\nrather than unique constraints. In that case, deferrable cannot be set.
includeLink to this heading#A list or tuple of the names of the fields to be included in the covering\nunique index as non-key columns. This allows index-only scans to be used for\nqueries that select only included fields (include)\nand filter only by unique fields (fields).
For example:
\nUniqueConstraint(name="unique_booking", fields=["room", "date"], include=["full_name"])\nwill allow filtering on room and date, also selecting full_name,\nwhile fetching data only from the index.
Unique constraints with non-key columns are ignored for databases besides\nPostgreSQL.
\nNon-key columns have the same database restrictions as Index.include.
opclassesLink to this heading#The names of the PostgreSQL operator classes to use for\nthis unique index. If you require a custom operator class, you must provide one\nfor each field in the index.
\nFor example:
\nUniqueConstraint(\n name="unique_username", fields=["username"], opclasses=["varchar_pattern_ops"]\n)\ncreates a unique index on username using varchar_pattern_ops.
opclasses are ignored for databases besides PostgreSQL.
nulls_distinctLink to this heading#Whether rows containing NULL values covered by the unique constraint should\nbe considered distinct from each other. The default value is None which\nuses the database default which is True on most backends.
For example:
\nUniqueConstraint(name="ordering", fields=["ordering"], nulls_distinct=False)\ncreates a unique constraint that only allows one row to store a NULL value\nin the ordering column.
Unique constraints with nulls_distinct are ignored for databases besides\nPostgreSQL.
violation_error_codeLink to this heading#The error code used when a ValidationError is raised during\nmodel validation.
Defaults to BaseConstraint.violation_error_code, when either\nUniqueConstraint.condition is set or UniqueConstraint.fields\nis not set.
If UniqueConstraint.fields is set without a\nUniqueConstraint.condition, defaults to the\nMeta.unique_together error\ncode when there are multiple fields, and to the Field.unique error\ncode when there is a single field.
violation_error_messageLink to this heading#The error message used when a ValidationError is raised during\nmodel validation.
Defaults to BaseConstraint.violation_error_message, when either\nUniqueConstraint.condition is set or UniqueConstraint.fields\nis not set.
If UniqueConstraint.fields is set without a\nUniqueConstraint.condition, defaults to the\nMeta.unique_together error\nmessage when there are multiple fields, and to the Field.unique error\nmessage when there is a single field.