docs: def transform_incoming(self, son, collection):
          """Replace embedded documents with DBRefs.
          """
  
          def transform_value(value):
              if isinstance(value, collections.MutableMapping):
                  if "_id" in value and "_ns" in value:
                      return DBRef(value["_ns"], transform_value(value["_id"]))
                  else:
                      return transform_dict(SON(value))
              elif isinstance(value, list):
                  return [transform_value(v) for v in value]
              return value
  
          def transform_dict(object):
              for (key, value) in object.items():
                  object[key] = transform_value(value)
              return object
  
          return transform_dict(SON(son))