docs: def sqlalchemy_validator(node, value, model, session=None, msg='%s is ongeldig'):
      """
      Validate a sqlalchemy object or identifier.
      Function checks if value is an instance of sqlalchemy model
      or an id of one.
      :param node: A colander SchemaNode.
      :param value: The value to be validated. Is the key of the
          SQLAlchemy model.
      :param model: A SQLAlchemy model.
      :param session: A SQLAlchemy session. Is required if value is not an
          instance of a SQLAlchemy model.
      :param msg: A msg to attach to a `colander.Invalid` exception.
      :raises colander.Invalid: If not a valid sqlalchemy model.
      """
  
      m = session.query(model).get(value)
  
      if not m:
          raise colander.Invalid(
              node,
              msg % value
          )
  
      return m