code: def _check_no_current_table(new_obj, current_table):
      
      if current_table is None:
          msg = 'Cannot add {} before adding table'
          if isinstance(new_obj, Relation):
              raise NoCurrentTableException(msg.format('relation'))
          if isinstance(new_obj, Column):
              raise NoCurrentTableException(msg.format('column'))
docs: Raises exception if we try to add a relation or a column
      with no current table.