many to many flask sqlalchemy

1association_table = db.Table('association', db.Model.metadata,
2    db.Column('left_id', db.Integer, db.ForeignKey('left.id')),
3    db.Column('right_id', db.Integer, db.ForeignKey('right.id'))
4)
5
6class Parent(db.Model):
7    __tablename__ = 'left'
8    id = db.Column(db.Integer, primary_key=True)
9    children = db.relationship("Child",
10                    secondary=association_table)
11
12class Child(db.Model):
13    __tablename__ = 'right'
14    id = db.Column(db.Integer, primary_key=True)
15
16
17p = Parent()
18c = Child()
19p.children.append(c)
20db.session.add(p)
21db.session.commit()

1student_identifier = db.Table('student_identifier',
2    db.Column('class_id', db.Integer, db.ForeignKey('classes.class_id')),
3    db.Column('user_id', db.Integer, db.ForeignKey('students.user_id'))
4)
5
6class Student(db.Model):
7    __tablename__ = 'students'
8    user_id = db.Column(db.Integer, primary_key=True)
9    user_fistName = db.Column(db.String(64))
10    user_lastName = db.Column(db.String(64))
11    user_email = db.Column(db.String(128), unique=True)
12
13
14class Class(db.Model):
15    __tablename__ = 'classes'
16    class_id = db.Column(db.Integer, primary_key=True)
17    class_name = db.Column(db.String(128), unique=True)
18    students = db.relationship("Student",
19                               secondary=student_identifier)
20
21s = Student()
22c = Class()
23c.students.append(s)
24db.session.add(c)
25db.session.commit()