docs: def schema(self):
          """return the schema instance if this is reference to another table"""
          if not hasattr(self, "_schema"):
              ret = None
              o = self._type
              if isinstance(o, type):
                  ret = getattr(o, "schema", None)
  
              elif isinstance(o, Schema):
                  ret = o
  
              else:
                  module, klass = utils.get_objects(o)
                  ret = klass.schema
  
              self._schema = ret
  
          return self._schema