code: def get_columns_list(self):
          
          columns = list(self.obj._fields.keys())
          if "_cls" in columns:
              columns.remove("_cls")
          return columns
docs: modified: removing the '_cls' column added by Mongoengine to support
          mongodb document inheritance
          cf. http://docs.mongoengine.org/apireference.html#documents:
          "A Document subclass may be itself subclassed,
          to create a specialised version of the document that will be
          stored in the same collection.
          To facilitate this behaviour a _cls field is added to documents
          (hidden though the MongoEngine interface).
          To disable this behaviour and remove the dependence on the presence of _cls set
          allow_inheritance to False in the meta dictionary."