      def __call__(self, value):
          queryset = self.queryset
          queryset = self.filter_queryset(value, queryset)
          queryset = self.exclude_current_instance(queryset)
          if queryset.exists():
              raise ValidationError(self.message)
  
      def __repr__(self):
          return unicode_to_repr('<%s(queryset=%s)>' % (
              self.__class__.__name__,
 
</s> remove         if queryset.exists():
 </s> add         if qs_exists(queryset): </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) </s> remove         return queryset.filter(**filter_kwargs)
 </s> add         return qs_filter(queryset, **filter_kwargs) </s> remove         return queryset.filter(**filter_kwargs)
 </s> add         return qs_filter(queryset, **filter_kwargs)
        if qs_exists(queryset):
      def __call__(self, value):
          queryset = self.queryset
          queryset = self.filter_queryset(value, queryset)
          queryset = self.exclude_current_instance(queryset)
        if qs_exists(queryset):
              raise ValidationError(self.message)
  
      def __repr__(self):
          return unicode_to_repr('<%s(queryset=%s)>' % (
              self.__class__.__name__,
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