本模块中定义的类可以创建数据库约束。它们被添加到模型中 Meta.constraints 选项中。
引用内置约束
约束是在 django.db.models.constraint 中定义的，但为了方便，它们被导入到 django.db.models 中。标准的惯例是使用 from django.db import models 作为 models.<Foo>Constraint。
抽象基类中的约束
你必须始终为约束指定一个唯一的名称。因此，你通常不能在抽象基类上指定一个约束，因为 Meta.craces 选项是由子类继承的，每次的属性值（包括 name）都完全相同。为了解决名称碰撞的问题，名称的一部分可能包含 '%(app_label)s' 和 '%(class)s'，它们分别被具体模型的小写应用标签和类名所代替。例如 CheckConstraint(check=Q(age__gte=18)，name='%(app_label)s_%(class)s_is_adult')。
约束的验证
Constraints are checked during the model validation.
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:
violation_error_message
The error message used when ValidationError is raised during
model validation. Defaults to
"Constraint “%(name)s” is violated.".
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
check
一个 Q 对象或布尔值 Expression，它指定了你要强制约束的检查。
例如，CheckConstraint(check=Q(age__gte=18), name='age_gte_18') 确保年龄字段永远不小于 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) 在数据库中创建一个唯一约束。
expressions
Positional argument *expressions allows creating functional unique
constraints on expressions and database functions.
例子：
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
一个字段名的列表，它指定了你要强制约束的唯一列集。
例如，UniqueConstraint(field=['room', 'date'], name='unique_booking') 确保每个房间在每个日期只能被预订一次。
condition
一个 Q 对象，用于指定你想要强制执行的约束条件。
例子：
UniqueConstraint(fields=['user'], condition=Q(status='DRAFT'), name='unique_draft_user')确保每个用户只有一份草稿。
这些条件与 Index.condition 具有相同的数据库限制。
deferrable
设置该参数，可创建一个可推迟的唯一约束。接受的值是 Deferrable.DEFERRED 或 Deferrable.IMMEDIATE。例如：
from django.db.models import Deferrable, UniqueConstraint
UniqueConstraint(
name='unique_order',
fields=['order'],
deferrable=Deferrable.DEFERRED,
)默认情况下，约束条件是不推迟的。推迟的约束条件在事务结束前不会被强制执行。即时约束将在每条命令后立即执行。
MySQL，MariaDB 和 SQLite。
在 MySQL、MariaDB 和 SQLite 上，可推迟的唯一约束被忽略，因为它们都不支持。
警告
推迟的唯一约束可能导致 性能惩罚 。
include
一个包含在覆盖的唯一索引中的字段名称的列表或元组，作为非键列。这允许只用索引扫描，用于只选择包含的字段（ include ）和只过滤唯一字段（ fields ）的查询。
例子：
UniqueConstraint(name='unique_booking', fields=['room', 'date'], include=['full_name'])将允许对 room 和 date 进行过滤，也可以选择 full_name，同时只从索引中获取数据。
include 只在 PostgreSQL 上支持。
Non-key columns have the same database restrictions as Index.include.
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.
例子：
UniqueConstraint(name='unique_username', fields=['username'], opclasses=['varchar_pattern_ops'])creates a unique index on username using varchar_pattern_ops.
opclasses 对于 PostgreSQL 以外的数据库来说是被忽略的。
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.
讨论区