Flask-Sqlalchemy, Primary key for secondary table in many-to-many relationship

Question

I am trying to build simple many-to-many relationship using Flask-Sqlalchemy for Postgresql Database:

from app import db
from sqlalchemy.dialects.postgresql import UUID

authors_books = db.Table(
    'authors_books',
    db.Column('id', UUID(as_uuid=True), primary_key=True),
    db.Column('author_id', UUID(as_uuid=True), db.ForeignKey('authors.id')),
    db.Column('book_id', UUID(as_uuid=True), db.ForeignKey('books.id')),
)

class Author(db.Model):
    __tablename__ = 'authors'
    # Fields
    id = db.Column(UUID(as_uuid=True), primary_key=True)
    first_name = db.Column(db.String)
    last_name = db.Column(db.String)

    # Relationships
    books = db.relationship('Book',
                            secondary=authors_books,
                            backref=db.backref('authors', lazy='dynamic'))

class Book(db.Model):
    __tablename__ = 'books'
    id = db.Column(UUID(as_uuid=True), primary_key=True)
    title = db.Column(db.String)

If i try to append Book to Author's books list:

u = User.query.get(user_id)
b = Book.query.get(book_id)
u.books.append(b)

So i've got this error:

sqlalchemy.exc.IntegrityError: (psycopg2.IntegrityError) null value in column "id" violates not-null constraint
DETAIL:  Failing row contains (null, "d5fb5e3f-2082-41f5-bcd2-bfdde11c78d4", "058fdbbd-d49f-4626-b669-b48dc4e75203").
[SQL: 'INSERT INTO authors_books (author_id, book_id) VALUES (%(author_id)s, %(book_id)s) RETURNING authors_books.id']
[parameters: {'author_id': UUID('d5fb5e3f-2082-41f5-bcd2-bfdde11c78d4'), 'book_id': UUID('058fdbbd-d49f-4626-b669-b48dc4e75203')}]
// Werkzeug Debugger

Is there way to configure models for setting primary key in association table automatically?

Answer 1

https://groups.google.com/forum/#!topic/sqlalchemy/WAHaPIuzqZQ

authors_books = db.Table(
    'authors_books',
    db.Column('id', UUID(as_uuid=True), primary_key=True, default=lambda: uuid.uuid4().hex),
    db.Column('author_id', UUID(as_uuid=True), db.ForeignKey('authors.id')),
    db.Column('book_id', UUID(as_uuid=True), db.ForeignKey('books.id')),
)