flask中一对一,一对多以及多对多关系的设定

class Grade(db.Model):
    id=db.Column(db.Integer,primary_key=True)
    name=db.Column(db.String(20),nullable=False)
    num = db.Column(db.Integer, default=0)
    time = db.Column(db.String(20), default=‘2018-01-01‘)

    students=db.relationship(‘Student‘,backref=‘gd‘,lazy=‘dynamic‘)
    __tablename__=‘grade‘


class Student(db.Model):
    id=db.Column(db.String(50),primary_key=True)
    name=db.Column(db.String(50),nullable=False)
    password = db.Column(db.String(50), nullable=False)
    sex = db.Column(db.String(50), nullable=False)
    phone = db.Column(db.String(11), nullable=False)
    photo = db.Column(db.String(100), default=None)

    grades=db.Column(db.Integer,db.ForeignKey(‘grade.id‘))

    __tablename__ = ‘student‘

t_g=db.Table(‘t_g‘,db.Column(‘t_id‘,db.String(50),db.ForeignKey(‘teacher.id‘)),db.Column(‘g_id‘,db.Integer,db.ForeignKey(‘grade.id‘)))

class Teacher(db.Model,DBO):
    id=db.Column(db.String(50),primary_key=True)
    name=db.Column(db.String(50),nullable=False)
    password = db.Column(db.String(50), nullable=False)
    phone=db.Column(db.String(11), nullable=False)
    photo=db.Column(db.String(100),default=None)

    grades=db.relationship(‘Grade‘,secondary=t_g,backref=‘th‘,lazy=‘dynamic‘)
    __tablename__=‘teacher‘


class Grade(db.Model,DBO):
    id=db.Column(db.Integer,primary_key=True)
    name=db.Column(db.String(20),nullable=False)
    num = db.Column(db.Integer, default=0)
    time = db.Column(db.String(20), default=‘2018-01-01‘)

    students=db.relationship(‘Student‘,backref=‘gd‘,lazy=‘dynamic‘)
    __tablename__=‘grade‘