fix: add model parents to create unique_constraint_by_fields

Author: s-aleshin

rest_framework/serializers.py

@@ -1607,7 +1607,9 @@ def get_unique_together_validators(self):
             source_map[source].append(name)
 
         unique_constraint_by_fields = {
-            constraint.fields: constraint for constraint in self.Meta.model._meta.constraints
+            constraint.fields: constraint
+            for model_cls in (self.Meta.model, *self.Meta.model._meta.parents)
+            for constraint in model_cls._meta.constraints
             if isinstance(constraint, models.UniqueConstraint)
         }
 

tests/test_validators.py

@@ -627,13 +627,7 @@ class Meta:
                 fields=("username", "company_id"),
                 name="unique_username_company_custom_msg",
                 violation_error_message="Username must be unique within a company.",
-                violation_error_code="duplicate_username",
-            )
-            if django_version[0] >= 5
-            else models.UniqueConstraint(
-                fields=("username", "company_id"),
-                name="unique_username_company_custom_msg",
-                violation_error_message="Username must be unique within a company.",
+                **(dict(violation_error_code="duplicate_username") if django_version[0] >= 5 else {}),
             ),
             models.UniqueConstraint(
                 fields=("company_id", "role"),