fix(serializer): restore get_unique_together_constraints method signature

Extracted error message logic to a separate method.

Author: s-aleshin

rest_framework/serializers.py

@@ -1427,33 +1427,20 @@ def get_extra_kwargs(self):
 
     def get_unique_together_constraints(self, model):
         """
-        Returns iterator of (fields, queryset, condition_fields, condition, message, code),
+        Returns iterator of (fields, queryset, condition_fields, condition),
         each entry describes an unique together constraint on `fields` in `queryset`
-        with respect of constraint's `condition`, optional custom `violation_error_message` and `violation_error_code`.
+        with respect of constraint's `condition`.
         """
         for parent_class in [model] + list(model._meta.parents):
             for unique_together in parent_class._meta.unique_together:
-                yield unique_together, model._default_manager, [], None, None, None
+                yield unique_together, model._default_manager, [], None
             for constraint in parent_class._meta.constraints:
                 if isinstance(constraint, models.UniqueConstraint) and len(constraint.fields) > 1:
                     if constraint.condition is None:
                         condition_fields = []
                     else:
                         condition_fields = list(get_referenced_base_fields_from_q(constraint.condition))
-
-                    violation_error_message = constraint.get_violation_error_message()
-                    default_error_message = constraint.default_violation_error_message % {"name": constraint.name}
-                    if violation_error_message == default_error_message:
-                        violation_error_message = None
-
-                    yield (
-                        constraint.fields,
-                        model._default_manager,
-                        condition_fields,
-                        constraint.condition,
-                        violation_error_message,
-                        getattr(constraint, "violation_error_code", None)
-                    )
+                    yield (constraint.fields, model._default_manager, condition_fields, constraint.condition)
 
     def get_uniqueness_extra_kwargs(self, field_names, declared_fields, extra_kwargs):
         """
@@ -1486,7 +1473,7 @@ def get_uniqueness_extra_kwargs(self, field_names, declared_fields, extra_kwargs
 
         # Include each of the `unique_together` and `UniqueConstraint` field names,
         # so long as all the field names are included on the serializer.
-        for unique_together_list, queryset, condition_fields, condition, *__ in self.get_unique_together_constraints(model):
+        for unique_together_list, queryset, condition_fields, condition in self.get_unique_together_constraints(model):
             unique_together_list_and_condition_fields = set(unique_together_list) | set(condition_fields)
             if model_fields_names.issuperset(unique_together_list_and_condition_fields):
                 unique_constraint_names |= unique_together_list_and_condition_fields
@@ -1582,6 +1569,17 @@ def get_validators(self):
             self.get_unique_for_date_validators()
         )
 
+    def _get_constraint_violation_error_message(self, constraint):
+        """
+        Returns the violation error message for the UniqueConstraint,
+        or None if the message is the default.
+        """
+        violation_error_message = constraint.get_violation_error_message()
+        default_error_message = constraint.default_violation_error_message % {"name": constraint.name}
+        if violation_error_message == default_error_message:
+            return None
+        return violation_error_message
+
     def get_unique_together_validators(self):
         """
         Determine a default set of validators for any unique_together constraints.
@@ -1608,10 +1606,15 @@ def get_unique_together_validators(self):
         for name, source in field_sources.items():
             source_map[source].append(name)
 
+        unique_constraint_by_fields = {
+            constraint.fields: constraint for constraint in self.Meta.model._meta.constraints
+            if isinstance(constraint, models.UniqueConstraint)
+        }
+
         # Note that we make sure to check `unique_together` both on the
         # base model class, but also on any parent classes.
         validators = []
-        for unique_together, queryset, condition_fields, condition, message, code in self.get_unique_together_constraints(self.Meta.model):
+        for unique_together, queryset, condition_fields, condition in self.get_unique_together_constraints(self.Meta.model):
             # Skip if serializer does not map to all unique together sources
             unique_together_and_condition_fields = set(unique_together) | set(condition_fields)
             if not set(source_map).issuperset(unique_together_and_condition_fields):
@@ -1634,13 +1637,17 @@ def get_unique_together_validators(self):
                 )
 
             field_names = tuple(source_map[f][0] for f in unique_together)
+
+            constraint = unique_constraint_by_fields.get(tuple(unique_together), None)
+            violation_error_message = self._get_constraint_violation_error_message(constraint) if constraint else None
+
             validator = UniqueTogetherValidator(
                 queryset=queryset,
                 fields=field_names,
                 condition_fields=tuple(source_map[f][0] for f in condition_fields),
                 condition=condition,
-                message=message,
-                code=code,
+                message=violation_error_message,
+                code=getattr(constraint, 'violation_error_code', None),
             )
             validators.append(validator)
         return validators