SQLAlchemy example many-to-many

"""
Пример работы в SQLAlchemy с двумя таблицами, связанными отношением многие ко многим.
"""

from sqlalchemy import Column, Integer, String, ForeignKey, create_engine, CheckConstraint
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker


engine = create_engine('postgresql+psycopg2://user:user@127.0.0.1/test')
Session = sessionmaker(bind=engine)

Base = declarative_base()

class Book(Base):
    __tablename__ = 'books'
    id = Column(Integer, primary_key=True)
    name = Column(String)
    rating = Column(Integer)
    __table_args__ = (CheckConstraint('rating >= 1 and rating <= 5', name='check_rating'), {})

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

    def __repr__(self):
        return "<Book(%s, '%s', %s)>" % (self.id, self.name, self.rating)

class Author(Base):
    __tablename__ = 'authors'
    id = Column(Integer, primary_key=True)
    name = Column(String)

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

    def __repr__(self):
        return "<Author(%s, '%s')>" % (self.id, self.name)

class AuthorToBook(Base):
    __tablename__ = 'authors_to_books'
    id = Column(Integer, primary_key=True)
    author_id = Column(Integer, ForeignKey('authors.id'))
    book_id = Column(Integer, ForeignKey('books.id'))

    def __init__(self, author, book):
        self.author_id = author
        self.book_id = book

    def __repr__(self):
        return "<AuthorToBook(id %s, author %s, book %s)>" % (self.id, self.author_id, self.book_id)

Base.metadata.create_all(engine)

session = Session()

# чистим таблицы перед вставкой (для тестов)
session.query(AuthorToBook).delete()
session.query(Author).delete()
session.query(Book).delete()
session.commit()

author1 = Author(name='Александр Сергеевич Пушкин')
author2 = Author(name='Михаил Юрьевич Лермонтов')
session.add_all([author1, author2])

session.commit()

print(*session.query(Author).all(), '', sep='\n')

book1 = Book(name='Сказка о царе Салтане', rating=5)
book2 = Book(name='Озорная классика для взрослых', rating=3)
book3 = Book(name='Герой нашего времени', rating=4)
session.add_all([book1, book2, book3])

session.commit()

print(*session.query(Book).all(), '', sep='\n')

publication1 = AuthorToBook(author=author1.id, book=book1.id)
publication2 = AuthorToBook(author=author1.id, book=book2.id)
publication3 = AuthorToBook(author=author2.id, book=book2.id)
publication4 = AuthorToBook(author=author2.id, book=book3.id)
session.add_all([publication1, publication2, publication3, publication4])

session.commit()

print(*session.query(AuthorToBook).all(), '', sep='\n')

"""
<Author(69, 'Александр Сергеевич Пушкин')>
<Author(70, 'Михаил Юрьевич Лермонтов')>

<Book(55, 'Сказка о царе Салтане', 5)>
<Book(56, 'Озорная классика для взрослых', 3)>
<Book(57, 'Герой нашего времени', 4)>

<AuthorToBook(id 49, author 69, book 55)>
<AuthorToBook(id 50, author 69, book 56)>
<AuthorToBook(id 51, author 70, book 56)>
<AuthorToBook(id 52, author 70, book 57)>
"""

print('done')