fix(serializer): restore get_unique_together_constraints method signature

Extracted error message logic to a separate method.

fix: conditionally include violation_error_code for Django >= 5.0

fix(validators): use custom error message and code from model constraints

rest_framework/serializers.py

@@ -1569,6 +1569,17 @@ class ModelSerializer(Serializer):
             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.
@@ -1595,6 +1606,11 @@ class ModelSerializer(Serializer):
         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 = []
@@ -1621,11 +1637,17 @@ class ModelSerializer(Serializer):
                 )
 
             field_names = tuple(source_map[f][0] for f in unique_together)
+
+            constraint = unique_constraint_by_fields.get(tuple(unique_together))
+            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=violation_error_message,
+                code=getattr(constraint, 'violation_error_code', None),
             )
             validators.append(validator)
         return validators

rest_framework/validators.py

@@ -111,13 +111,15 @@ class UniqueTogetherValidator:
     message = _('The fields {field_names} must make a unique set.')
     missing_message = _('This field is required.')
     requires_context = True
+    code = 'unique'
 
-    def __init__(self, queryset, fields, message=None, condition_fields=None, condition=None):
+    def __init__(self, queryset, fields, message=None, condition_fields=None, condition=None, code=None):
         self.queryset = queryset
         self.fields = fields
         self.message = message or self.message
         self.condition_fields = [] if condition_fields is None else condition_fields
         self.condition = condition
+        self.code = code or self.code
 
     def enforce_required_fields(self, attrs, serializer):
         """
@@ -198,7 +200,7 @@ class UniqueTogetherValidator:
         if checked_values and None not in checked_values and qs_exists_with_condition(queryset, self.condition, condition_kwargs):
             field_names = ', '.join(self.fields)
             message = self.message.format(field_names=field_names)
-            raise ValidationError(message, code='unique')
+            raise ValidationError(message, code=self.code)
 
     def __repr__(self):
         return '<{}({})>'.format(
@@ -217,6 +219,7 @@ class UniqueTogetherValidator:
                 and self.missing_message == other.missing_message
                 and self.queryset == other.queryset
                 and self.fields == other.fields
+                and self.code == other.code
                 )
 
 

tests/test_validators.py

@@ -616,6 +616,32 @@ class UniqueConstraintNullableModel(models.Model):
         ]
 
 
+class UniqueConstraintCustomMessageCodeModel(models.Model):
+    username = models.CharField(max_length=32)
+    company_id = models.IntegerField()
+    role = models.CharField(max_length=32)
+
+    class Meta:
+        constraints = [
+            models.UniqueConstraint(
+                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.",
+            ),
+            models.UniqueConstraint(
+                fields=("company_id", "role"),
+                name="unique_company_role_default_msg",
+            ),
+        ]
+
+
 class UniqueConstraintSerializer(serializers.ModelSerializer):
     class Meta:
         model = UniqueConstraintModel
@@ -628,6 +654,12 @@ class UniqueConstraintNullableSerializer(serializers.ModelSerializer):
         fields = ('title', 'age', 'tag')
 
 
+class UniqueConstraintCustomMessageCodeSerializer(serializers.ModelSerializer):
+    class Meta:
+        model = UniqueConstraintCustomMessageCodeModel
+        fields = ('username', 'company_id', 'role')
+
+
 class TestUniqueConstraintValidation(TestCase):
     def setUp(self):
         self.instance = UniqueConstraintModel.objects.create(
@@ -778,6 +810,31 @@ class TestUniqueConstraintValidation(TestCase):
         )
         assert serializer.is_valid()
 
+    def test_unique_constraint_custom_message_code(self):
+        UniqueConstraintCustomMessageCodeModel.objects.create(username="Alice", company_id=1, role="member")
+        expected_code = "duplicate_username" if django_version[0] >= 5 else UniqueTogetherValidator.code
+
+        serializer = UniqueConstraintCustomMessageCodeSerializer(data={
+            "username": "Alice",
+            "company_id": 1,
+            "role": "admin",
+        })
+        assert not serializer.is_valid()
+        assert serializer.errors == {"non_field_errors": ["Username must be unique within a company."]}
+        assert serializer.errors["non_field_errors"][0].code == expected_code
+
+    def test_unique_constraint_default_message_code(self):
+        UniqueConstraintCustomMessageCodeModel.objects.create(username="Alice", company_id=1, role="member")
+        serializer = UniqueConstraintCustomMessageCodeSerializer(data={
+            "username": "John",
+            "company_id": 1,
+            "role": "member",
+        })
+        expected_message = UniqueTogetherValidator.message.format(field_names=', '.join(("company_id", "role")))
+        assert not serializer.is_valid()
+        assert serializer.errors == {"non_field_errors": [expected_message]}
+        assert serializer.errors["non_field_errors"][0].code == UniqueTogetherValidator.code
+
 
 # Tests for `UniqueForDateValidator`
 # ----------------------------------