code: def _on_event(self, event):
          
          if self.has_option(event):
              on_event = self.get_option(event).upper()
  
              if on_event not in ["NO ACTION", "RESTRICT"]:
                  return on_event
  
          return False
docs: Returns the referential action for a given database operation
          on the referenced table the foreign key constraint is associated with.
  
          :param event: Name of the database operation/event to return the referential action for.
          :type event: str
  
          :rtype: str or None