orm 对象创建
orm 创建时,如果是对象则用字段=对象,否则实用字段_id= id, student=student, student_id=student_id, 如果用student=student_id则报错must be a "Student" instance.
orm 创建时,如果是对象则用字段=对象,否则实用字段_id= id, student=student, student_id=student_id, 如果用student=student_id则报错must be a "Student" instance.