docs: def get_instance(model, instance_id):
      """Get an instance by id.
  
      :param model: a string, model name in rio.models
      :param id: an integer, instance id.
      :return: None or a SQLAlchemy Model instance.
      """
      try:
          model = get_model(model)
      except ImportError:
          return None
  
      return model.query.get(instance_id)