Avoid unnecessary unique together checking (#9154)

Author: yuekui

rest_framework/validators.py

@@ -160,10 +160,19 @@ def __call__(self, attrs, serializer):
         queryset = self.exclude_current_instance(attrs, queryset, serializer.instance)
 
         # Ignore validation if any field is None
-        checked_values = [
-            value for field, value in attrs.items() if field in self.fields
-        ]
-        if None not in checked_values and qs_exists(queryset):
+        if serializer.instance is None:
+            checked_values = [
+                value for field, value in attrs.items() if field in self.fields
+            ]
+        else:
+            # Ignore validation if all field values are unchanged
+            checked_values = [
+                value
+                for field, value in attrs.items()
+                if field in self.fields and value != getattr(serializer.instance, field)
+            ]
+
+        if checked_values and None not in checked_values and qs_exists(queryset):
             field_names = ', '.join(self.fields)
             message = self.message.format(field_names=field_names)
             raise ValidationError(message, code='unique')

tests/test_validators.py

@@ -1,5 +1,5 @@
 import datetime
-from unittest.mock import MagicMock
+from unittest.mock import MagicMock, patch
 
 import pytest
 from django.db import DataError, models
@@ -447,6 +447,22 @@ def test_do_not_ignore_validation_for_null_fields(self):
         serializer = NullUniquenessTogetherSerializer(data=data)
         assert not serializer.is_valid()
 
+    def test_ignore_validation_for_unchanged_fields(self):
+        """
+        If all fields in the unique together constraint are unchanged,
+        then the instance should skip uniqueness validation.
+        """
+        instance = UniquenessTogetherModel.objects.create(
+            race_name="Paris Marathon", position=1
+        )
+        data = {"race_name": "Paris Marathon", "position": 1}
+        serializer = UniquenessTogetherSerializer(data=data, instance=instance)
+        with patch(
+            "rest_framework.validators.qs_exists"
+        ) as mock:
+            assert serializer.is_valid()
+            assert not mock.called
+
     def test_filter_queryset_do_not_skip_existing_attribute(self):
         """
         filter_queryset should add value from existing instance attribute