id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
35638	validate_constraints() fails on models with fields using db_default	David Sanders	David Sanders	"Follow up to #35223

Related forum thread: https://forum.djangoproject.com/t/proposal-to-have-db-default-considered-in-model-forms/33358/3

Given the model

{{{
class Foo(models.Model):
    bar = models.CharField(db_default=""bar"")

    class Meta:
        constraints = [
            models.CheckConstraint(check=models.Q(bar=""bar""), name=""is_bar""),
        ]
}}}

validating constraints should ideally not fail but currently does:

{{{
>>> foo = Foo()
>>> foo.validate_constraints()
Traceback (most recent call last):
  File ""<console>"", line 1, in <module>
  File ""/path/to/django/django/db/models/base.py"", line 1598, in validate_constraints
    raise ValidationError(errors)
django.core.exceptions.ValidationError: {'__all__': ['Constraint “is_bar” is violated.']}
}}}

I couldn't find an easy workaround for this.

If I apply this small patch to `Q.check()`, then `validate_constraints()` uses the default expression and runs a suitable check query. Perhaps can take this as a starting point and add something similar to Django?

{{{
index 1bf396723e..5380cc17d0 100644
--- a/django/db/models/query_utils.py
+++ b/django/db/models/query_utils.py
@@ -120,12 +120,16 @@ class Q(tree.Node):
         """"""
         # Avoid circular imports.
         from django.db.models import BooleanField, Value
+        from django.db.models.expressions import DatabaseDefault
         from django.db.models.functions import Coalesce
         from django.db.models.sql import Query
         from django.db.models.sql.constants import SINGLE

         query = Query(None)
         for name, value in against.items():
+            # not ideal, value is wrapped in a Value by Model._get_field_value_map()
+            if isinstance(getattr(value, ""value""), DatabaseDefault):
+                value = value.field.db_default
             if not hasattr(value, ""resolve_expression""):
                 value = Value(value)
             query.add_annotation(value, name, select=False)
}}}"	Bug	closed	Database layer (models, ORM)	5.0	Release blocker	fixed	db_default		Ready for checkin	1	0	0	0	0	0