          elif field_name in info.relations:
              if not nested_depth:
                  return self.build_relational_field(field_name, info, model)
              else:
                  return self.build_nested_field(field_name, info, model, nested_depth)
  
          elif hasattr(model, field_name):
              return self.build_property_field(field_name, info, model)
  
          elif field_name == api_settings.URL_FIELD_NAME:
 
</s> remove                 return self.build_relational_field(field_name, info, model)
 </s> add                 return self.build_relational_field(field_name, relation_info) </s> remove             return self.build_standard_field(field_name, info, model)
 </s> add             model_field = info.fields_and_pk[field_name]
            return self.build_standard_field(field_name, model_field) </s> remove     def build_field(self, field_name, info, model, nested_depth):
 </s> add     def build_field(self, field_name, info, model_class, nested_depth): </s> remove         return field_cls, kwargs
 </s> add         return field_class, field_kwargs
                return self.build_nested_field(field_name, relation_info, nested_depth)
          elif field_name in info.relations:
              if not nested_depth:
                  return self.build_relational_field(field_name, info, model)
              else:
                return self.build_nested_field(field_name, relation_info, nested_depth)
  
          elif hasattr(model, field_name):
              return self.build_property_field(field_name, info, model)
  
          elif field_name == api_settings.URL_FIELD_NAME:
keep keep keep keep replace keep keep keep keep keep
 Final bits of docs for ModelSerializer fields API 
rest_framework/serializers.py