docs: def add_instance(model, _commit=True, **kwargs):
      """Add instance to database.
  
      :param model: a string, model name in rio.models
      :param _commit: control whether commit data to database or not. Default True.
      :param \*\*kwargs: persisted data.
      :return: instance id.
      """
      try:
          model = get_model(model)
      except ImportError:
          return None
  
      instance = model(**kwargs)
      db.session.add(instance)
  
      try:
          if _commit:
              db.session.commit()
          else:
              db.session.flush()
          return instance.id
      except IntegrityError:
          db.session.rollback()
          return