Flask-sqlalchemy many-to-many relationship

from app import db
 
 
us_ph_sub = db.Table('us_ph_sub',                             
  db.Column('user_id', db.Integer,db.ForeignKey("user.id"), nullable=False),
  db.Column('phone_id', db.Integer,db.ForeignKey("phone.id"), nullable=False),
  db.Column('subdivision_id', db.Integer,db.ForeignKey("subdivision.id"), nullable=False)
  )
                                      
                                      
class User(db.Model):
  id = db.Column(db.Integer, primary_key = True)
  surname = db.Column(db.String(64), index = True)
  name = db.Column(db.String(64), index = True)
  patronymic = db.Column(db.String(64), index = True)
  info = db.Column(db.String(1024))
 
  def __repr__(self):
    return '<User %r %r>' % (self.surname, self.name)
        
 
class Phone(db.Model):
  id = db.Column(db.Integer, primary_key = True)
  number = db.Column(db.Integer, unique = True, nullable=False)
 
  def __repr__(self):
    return '<Number %r>' % (self.number)
 
 
class Subdivision(db.Model):
  id = db.Column(db.Integer, primary_key = True)
  unit = db.Column(db.String(128), unique = True, nullable=False)
  
  def __repr__(self):
    return '<Unit %r>' % (self.unit)