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