Self referential many to many relationship with extended data, containing foreign key

v4.py

from sqlalchemy import create_engine
from sqlalchemy import Table, Column, Integer, ForeignKey, String
from sqlalchemy.orm import relationship, mapper, sessionmaker
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.ext.associationproxy import association_proxy

Base = declarative_base()

class User(Base):
    __tablename__ = 'users'
    id = Column(Integer, primary_key=True)
    username = Column(String)

    followers = association_proxy("leader_follower", "follower")
    leaders = association_proxy("follower_leader", "leader")

    def __init__(self, name):
        self.username = name

    def __repr__(self):
        return f"<User {self.username}>"

class Movement(Base):
    __tablename__ = 'movements'
    id = Column(Integer, primary_key=True)

class Association(Base):
    __tablename__ = 'assoc'

    id = Column(Integer, primary_key=True)
    leader_id = Column(Integer, ForeignKey('users.id'))
    follower_id = Column(Integer, ForeignKey('users.id'))
    movement_id = Column(Integer, ForeignKey('movements.id'))

    movement = relationship(Movement)
    leader = relationship(User, foreign_keys=[leader_id], backref="leader_follower")
    follower =  relationship(User, foreign_keys=[follower_id], backref="follower_leader")

    def __init__(self, leader, follower, movement):
        self.leader = leader
        self.follower = follower
        self.movement = movement


if __name__=='__main__':
    user1 = User('Alice')
    user2 = User('Bob')
    movement = Movement()
    a = Association(user1, user2, movement)
    print(a.leader)
    print(user1.followers)
    print(user2.leaders)