关于表之间多对多的关系，我现在有user表 和 role 还有一张user_role的中间表
user模型如下

from app.models.base import Base, db, MixinJSONSerializer
from app.models.user_role import user_role
from app.models.role import Role


class User(Base):
    __tablename__ = 'user'
    id = db.Column(db.Integer, primary_key=True, autoincrement=True)
    username = db.Column(db.String(50))
    realname = db.Column(db.String(50))
    avatar = db.Column(db.String(255))
    phone = db.Column(db.String(15))
    password = db.Column(db.String(255))
    create_time = db.Column(db.DateTime)
    update_time = db.Column(db.DateTime)
    user_group_id = db.Column(db.Integer, name='usergroup_id')

    role = db.relationship('Role', secondary=user_role, backref='user')

    def keys(self):
        return ['id', 'username', 'realname', 'avatar', 'phone', 'user_group_id']

user表中我主要导入了 Role模型，与中间user_role的table

role模型如下

from app import db
from app.models.base import Base


class Role(Base):
    __tablename__ = 'role'
    id = db.Column(db.Integer, primary_key=True, autoincrement=True)
    name = db.Column(db.String(32), index=True, nullable=True)

user_role模型如下

from app.models.base import db


user_role = db.Table('user_role',
                     db.Column('user_id', db.Integer, db.ForeignKey('user.id'), primary_key=True),
                     db.Column('role_id', db.Integer, db.ForeignKey('role.id'), primary_key=True)
                     )

这是我查找的 SQLAlchemy 0.9 Documentation 的相关文档

当我注释掉user模型中的 from app.models.role import Role 就会报错

这是注释掉导入Role之后的代码

from app.models.base import Base, db, MixinJSONSerializer
from app.models.user_role import user_role
# from app.models.role import Role


class User(Base):
    __tablename__ = 'user'
    id = db.Column(db.Integer, primary_key=True, autoincrement=True)
    username = db.Column(db.String(50))
    realname = db.Column(db.String(50))
    avatar = db.Column(db.String(255))
    phone = db.Column(db.String(15))
    password = db.Column(db.String(255))
    create_time = db.Column(db.DateTime)
    update_time = db.Column(db.DateTime)
    user_group_id = db.Column(db.Integer, name='usergroup_id')

    role = db.relationship('Role', secondary=user_role, backref='user')

    def keys(self):
        return ['id', 'username', 'realname', 'avatar', 'phone', 'user_group_id']

当然如果正常导入Role模型是可以正常查询的，我想问的是，我user模型中并没有用到role模型，为何不导入会报这种错误呢