Check out example codes for "many to many flask-sqlalchemy". It will help you in understanding the concepts better.

Code Example 1

association_table = db.Table('association', db.Model.metadata,
    db.Column('left_id', db.Integer, db.ForeignKey('left.id')),
    db.Column('right_id', db.Integer, db.ForeignKey('right.id'))
)

class Parent(db.Model):
    __tablename__ = 'left'
    id = db.Column(db.Integer, primary_key=True)
    children = db.relationship("Child",
                    secondary=association_table)

class Child(db.Model):
    __tablename__ = 'right'
    id = db.Column(db.Integer, primary_key=True)


p = Parent()
c = Child()
p.children.append(c)
db.session.add(p)
db.session.commit()

Code Example 2

student_identifier = db.Table('student_identifier',
    db.Column('class_id', db.Integer, db.ForeignKey('classes.class_id')),
    db.Column('user_id', db.Integer, db.ForeignKey('students.user_id'))
)

class Student(db.Model):
    __tablename__ = 'students'
    user_id = db.Column(db.Integer, primary_key=True)
    user_fistName = db.Column(db.String(64))
    user_lastName = db.Column(db.String(64))
    user_email = db.Column(db.String(128), unique=True)


class Class(db.Model):
    __tablename__ = 'classes'
    class_id = db.Column(db.Integer, primary_key=True)
    class_name = db.Column(db.String(128), unique=True)
    students = db.relationship("Student",
                               secondary=student_identifier)

s = Student()
c = Class()
c.students.append(s)
db.session.add(c)
db.session.commit()

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