# Django 5.2.8 documentation

# 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(condition=Q(age__gte=18), name="%(app_label)s_%(class)s_is_adult")

Validation of Constraints

Constraints are checked during the model validation.

## `BaseConstraint`

class BaseConstraint(*name, violation_error_code=None, 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_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” 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(*, condition, name, violation_error_code=None, violation_error_message=None)

  Creates a check constraint in the database.

### `condition`

CheckConstraint.condition

A `Q` object or boolean `Expression` that specifies the conditional check you want the constraint to enforce.

For example:

    CheckConstraint(condition=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(
        condition=Q(age__gte=18) & Q(expensive_check=condition),
        name="age_gte_18_and_others",
    )

Oracle < 23c

Checks with nullable fields on Oracle < 23c 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(condition=Q(age__gte=18) | Q(age__isnull=True), name="age_gte_18")

## `UniqueConstraint`

class UniqueConstraint(*expressions, fields=(), name=None, condition=None, deferrable=None, include=None, opclasses=(), nulls_distinct=None, violation_error_code=None, 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 they do not support 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.

### `violation_error_code`

UniqueConstraint.violation_error_code

The error code used when a `ValidationError` is raised during model validation.

Defaults to `BaseConstraint.violation_error_code`, when either `UniqueConstraint.condition` is set or `UniqueConstraint.fields` is not set.

If `UniqueConstraint.fields` is set without a `UniqueConstraint.condition`, defaults to the `Meta.unique_together` error code when there are multiple fields, and to the `Field.unique` error code when there is a single field.

### `violation_error_message`

UniqueConstraint.violation_error_message

The error message used when a `ValidationError` is raised during model validation.

Defaults to `BaseConstraint.violation_error_message`, when either `UniqueConstraint.condition` is set or `UniqueConstraint.fields` is not set.

If `UniqueConstraint.fields` is set without a `UniqueConstraint.condition`, defaults to the `Meta.unique_together` error message when there are multiple fields, and to the `Field.unique` error message when there is a single field.