          # If an element in `fields` is a 2-tuple of (str, tuple)
          # then the second element of the tuple is the fields to
          # set on the related serializer
          if isinstance(info, (list, tuple)):
              class OnTheFlySerializer(self.__class__):
                  fields = info
                  parent = getattr(self, 'parent') or self
              return OnTheFlySerializer
  
          # If an element in `fields` is a 2-tuple of (str, Serializer)
          # then the second element of the tuple is the Serializer
          # class to use for that field.
 
</s> remove         # Otherwise use `related_serializer` or fall back to `Serializer`
        return getattr(self, 'related_serializer') or Serializer
 </s> add         # Otherwise use `related_serializer` or fall back to
        # `OnTheFlySerializer` preserve custom serialization methods.
        return getattr(self, 'related_serializer') or OnTheFlySerializer
          # If an element in `fields` is a 2-tuple of (str, tuple)
          # then the second element of the tuple is the fields to
          # set on the related serializer
          if isinstance(info, (list, tuple)):



              return OnTheFlySerializer
  
          # If an element in `fields` is a 2-tuple of (str, Serializer)
          # then the second element of the tuple is the Serializer
          # class to use for that field.
keep keep keep keep replace replace replace keep keep keep keep keep
 Don't ever return the normal serializer again. 
djangorestframework/serializer.py