NoReferencedTableError: Foreign key associated with column ** with which to generate a foreign key to target column 'id'

1.使用 python flask 框架做项目时，在实体类中配置了 映射关系，

message:

id = db.Column(db.Integer, primary_key=True)
message_type_id = db.Column(db.Integer, db.ForeignKey('message_type.id'))

message_type:

_tablename__ = 'message_type'
id = db.Column(db.Integer, primary_key=True,autoincrement=True)
content = db.Column(db.String(1500))
访问到查询方法时一直报错：

NoReferencedTableError: Foreign key associated with column 'message.message_type_id' could not find table 'message_type' with which to generate a foreign key to target column 'id'

通过调试跟踪代码，发现 message_type 类没有实例化，所以在调用main.py 启动项目时，一定要加载到message_type 将它写入 mod_*中，实例化