'Q' object has no attribute 'replace_expressions'

[Petr Přikryl]

Hi, I have problem with calling full_clean on Model object with following constraint containing Q:

models.UniqueConstraint(
    models.Q(color__isnull=True),
    "product",
    condition=models.Q(color__isnull=True),
    name="unique_product_color_null",
),

Constraint is allowing only one NULL and works well. More at ​https://www.enterprisedb.com/postgres-tutorials/postgresql-unique-constraint-null-allowing-only-one-null

Traceback

/usr/local/lib/python3.11/site-packages/django/db/models/base.py:1497: in full_clean
    self.validate_constraints(exclude=exclude)
/usr/local/lib/python3.11/site-packages/django/db/models/base.py:1445: in validate_constraints
    constraint.validate(model_class, self, exclude=exclude, using=using)

/usr/local/lib/python3.11/site-packages/django/db/models/constraints.py:346: AttributeError
>               expressions.append(Exact(expr, expr.replace_expressions(replacements)))
E               AttributeError: 'Q' object has no attribute 'replace_expressions'

​https://github.com/django/django/blob/4.2.4/django/db/models/constraints.py#L346

[Mariusz Felisiak]

Your constraint definition is invalid, you cannot pass Q objects in expressions, check out ​docs. If you're having trouble understanding how Django works, see TicketClosingReasons/UseSupportChannels for ways to get help.