删除多对多：Flask-Sqlalchemy 尝试删除具有特定 id 的每一行

Question

我一直坚持使用烧瓶和烧瓶-sqlalchemy 从 MySQL 数据库中删除多对多记录。 好像 sqlalchemy 试图从关系表中删除所有行，其中 a 的 id 与链接到 b 的相同。

错误（示例）：

sqlalchemy.orm.exc.StaleDataError: DELETE statement on table 'ksiazka_autor' expected to delete 1 row(s); Only 0 were matched.

或

sqlalchemy.orm.exc.StaleDataError: DELETE statement on table 'ksiazka_wydawnictwo' expected to delete 552 row(s); Only 551 were matched.

INFO sqlalchemy.engine.base.Engine DELETE FROM ksiazka_kategoria WHERE ksiazka_kategoria.ksiazka_id = %s AND ksiazka_kategoria.kategoria_id = %s
2017-08-29 15:44:34,965 INFO sqlalchemy.engine.base.Engine ((166, 40), (167, 40), (168, 40), (169, 40), (170, 40), (171, 40), (172, 40), (173, 40)  ... displaying 10 of 178 total bound parameter sets ...  (1463, 19), (2046, 19))

关系表：

ksiazka_autor = db.Table('ksiazka_autor',
        db.Column('autor_id', db.Integer, db.ForeignKey('autor.id')),
        db.Column('ksiazka_id', db.Integer, db.ForeignKey('ksiazka.id'))
    )
        ksiazka_wydawnictwo = db.Table('ksiazka_wydawnictwo',
        db.Column('wydawnictwo_id', db.Integer, db.ForeignKey('wydawnictwo.id')),
        db.Column('ksiazka_id', db.Integer, db.ForeignKey('ksiazka.id'))
    )
    ksiazka_ilustrator = db.Table('ksiazka_ilustrator',
        db.Column('ilustrator_id', db.Integer, db.ForeignKey('ilustrator.id')),
        db.Column('ksiazka_id', db.Integer, db.ForeignKey('ksiazka.id'))
    )
    ksiazka_kategoria = db.Table('ksiazka_kategoria',
        db.Column('ksiazka_id', db.Integer, db.ForeignKey('ksiazka.id')),
        db.Column('kategoria_id', db.Integer, db.ForeignKey('kategoria.id'))
    )

和模型：

class Book(db.Model):
    __tablename__ = 'ksiazka'
    id = db.Column('id', db.Integer, primary_key=True)
    [...]

    authors = db.relationship('Author', secondary=ksiazka_autor, backref='authors', cascade="all", lazy='dynamic')
    category = db.relationship('Category', secondary=ksiazka_kategoria, backref='category', cascade="all", lazy='dynamic')
    publisher = db.relationship('Publisher', secondary=ksiazka_wydawnictwo, backref='publisher', cascade="all", lazy='dynamic')
    artist = db.relationship('Artist', secondary=ksiazka_ilustrator, backref='artist', cascade="all", lazy='dynamic')

class Author(db.Model):
    __tablename__ = 'autor'
    id = db.Column('id', db.Integer, primary_key=True)
    [...]
    books = db.relationship('Book', secondary=ksiazka_autor, backref=db.backref('books'), lazy='joined')

class Category(db.Model):
    __tablename__ = 'kategoria'
    id = db.Column('id', db.Integer, primary_key=True)
    [...]
    cat_books = db.relationship('Book', secondary=ksiazka_kategoria, backref=db.backref('cat_books'), lazy='joined')

class Publisher(db.Model):
    __tablename__ = 'wydawnictwo'
    id = db.Column('id', db.Integer, primary_key=True)
    [...]
    pub_books = db.relationship('Book', secondary=ksiazka_wydawnictwo, backref=db.backref('pub_books'), lazy='joined')

class Artist(db.Model):
    __tablename__ = 'ilustrator'
    id = db.Column('id', db.Integer, primary_key=True)
    [...]
    art_books = db.relationship('Book', secondary=ksiazka_ilustrator, backref=db.backref('art_books'), lazy='joined')

最后是烧瓶代码：

@app.route('/delete/book/<id>')
def delete_book(id):
    book = Book.query.filter_by(id=id).first()
    db.session.delete(book)
    db.session.commit()
    return redirect('/home')

Answer 1

这是因为您已为所需的每个关系设置了两个单独的关系。您有一个名为Book.authors 的关系和一个名为Author.authors 的反向引用，以及一个名为Author.books 的关系和一个名为Book.books 的反向引用。这不起作用，因为它们都在ksiazka_autor 上运行，从而导致您看到的冲突。您的所有其他关系也是如此。

您想要的是使用back_populates 选项：

class Book(db.Model):
    ...

    authors = db.relationship('Author', secondary=ksiazka_autor, back_populates='books', cascade="all", lazy='dynamic')

class Author(db.Model):
    ...
    books = db.relationship('Book', secondary=ksiazka_autor, back_populates='authors', lazy='joined')

手动删除相关实例并提交两次不是正确的解决方案。