Revert "Avoid unnecessary unique together checking (encode#9154)"

This reverts commit 41edb3b.

Tests not included.

Author: gergosimonyi

rest_framework/validators.py

@@ -174,22 +174,11 @@ def __call__(self, attrs, serializer):
         queryset = self.filter_queryset(attrs, queryset, serializer)
         queryset = self.exclude_current_instance(attrs, queryset, serializer.instance)
 
-        checked_names = [
-            serializer.fields[field_name].source for field_name in self.fields
-        ]
         # Ignore validation if any field is None
-        if serializer.instance is None:
-            checked_values = [attrs[field_name] for field_name in checked_names]
-        else:
-            # Ignore validation if all field values are unchanged
-            checked_values = [
-                attrs[field_name]
-                for field_name in checked_names
-                if attrs[field_name] != getattr(serializer.instance, field_name)
-            ]
-
-        condition_kwargs = {source: attrs[source] for source in self.condition_fields}
-        if checked_values and None not in checked_values and qs_exists_with_condition(queryset, self.condition, condition_kwargs):
+        checked_values = [
+            value for field, value in attrs.items() if field in self.fields
+        ]
+        if 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')