          filter_kwargs = {
              field_name: attrs[field_name]
              for field_name in self.fields
          }
          return queryset.filter(**filter_kwargs)
  
      def exclude_current_instance(self, attrs, queryset):
          """
          If an instance is being updated, then do not include
          that instance itself as a uniqueness conflict.
 
</s> remove         return queryset.filter(**filter_kwargs)
 </s> add         return qs_filter(queryset, **filter_kwargs) </s> remove         if None not in checked_values and queryset.exists():
 </s> add         if None not in checked_values and qs_exists(queryset): </s> remove         return queryset.filter(**filter_kwargs)
 </s> add         return qs_filter(queryset, **filter_kwargs)
        return qs_filter(queryset, **filter_kwargs)
          filter_kwargs = {
              field_name: attrs[field_name]
              for field_name in self.fields
          }
        return qs_filter(queryset, **filter_kwargs)
  
      def exclude_current_instance(self, attrs, queryset):
          """
          If an instance is being updated, then do not include
          that instance itself as a uniqueness conflict.
keep keep keep keep replace keep keep keep keep keep
 Robust uniqueness checks. (#4217)

* Robust uniqueness checks
* Add master to test matrix (allow_failures) 
rest_framework/validators.py