Many-to-Many between Category and Book

from flask import Flask
from flask.ext.sqlalchemy import SQLAlchemy

from sqlalchemy import ForeignKey, UniqueConstraint, DateTime
from sqlalchemy.orm import relationship, backref

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:////tmp/test.db'
db = SQLAlchemy(app)


class User(db.Model):
    __tablename__ = 'user'
    pk_id = db.Column(db.Integer, primary_key=True)
    active = db.Column(db.Boolean(), nullable=False, default=False)
    username = db.Column(db.String(50), nullable=False, unique=True)
    password = db.Column(db.String(255), nullable=False, default='')


class BookCategoryLink(db.Model):
    __tablename__ = 'book_category_link'
    book_id = db.Column(
        db.Integer, ForeignKey('book.pk_id'), primary_key=True
    )
    category_id = db.Column(
        db.Integer, ForeignKey('category.pk_id'), primary_key=True
    )


class Category(db.Model):
    __tablename__ = 'category'
    pk_id = db.Column(db.Integer, primary_key=True, unique=True)
    category_title = db.Column(db.String(50))
    books = relationship(
        'Book', secondary=BookCategoryLink.__table__,
        backref=backref('categories')
    )

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


class Book(db.Model):
    __tablename__ = 'book'
    pk_id = db.Column(db.Integer, primary_key=True)
    book_name = db.Column(db.String(256))

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


class Rating(db.Model):
    __tablename__ = 'rating'
    __table_args__ = (UniqueConstraint('user_id', 'book_id'),)

    id = db.Column(db.Integer, primary_key=True)
    user_id = db.Column(db.Integer, ForeignKey('user.pk_id'))
    book_id = db.Column(db.Integer, ForeignKey('book.pk_id'))

    user = relationship('User', backref=backref('ratings'))
    book = relationship('Book', backref=backref('ratings'))

    # genre, mood, plot, etc.
    genre = db.Column(db.String(50))
    plot = db.Column(db.String(50))
    mood = db.Column(db.String(50))
    pub_date = db.Column(db.DateTime)


db.create_all()


user = User(active=True, username='test', password='test')
db.session.add(user)
horror = Category('Horror')
fantasy = Category('Fantasy')
the_shining = Book('The Shining')
the_shining.categories.append(horror)
the_shining.categories.append(fantasy)
db.session.add(the_shining)
rating = Rating(
    user=user, book=the_shining,
)
db.session.add(rating)
db.session.commit()
db.session.close()