python - Flask - SQLAlchemy 多对多原因 sqlalchemy.orm.exc.FlushError

Question

class Show(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    title = db.Column(db.String(200), unique=True, nullable=False)
    studio = db.relationship("ShowStudio", backref="show", cascade="delete")

class Studio(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(100), unique=True, nullable=False)
    show = db.relationship("ShowStudio", backref="studio", cascade="delete")

class ShowStudio(db.Model):
    show_id = db.Column(db.Integer, db.ForeignKey(Show.id, ondelete="CASCADE"), primary_key=True)
    studio_id = db.Column(db.Integer, db.ForeignKey(Studio.id, ondelete="CASCADE"), primary_key=True)

studio = Studio(name="asd")

db.session.add(studio)
s = Show(title="a", studio=[studio])
db.session.flush()

db.session.add(s)
db.session.commit()

当我运行它时，它抛出

sqlalchemy.orm.exc.FlushError: Attempting to flush an item of type as a member of collection "Studio.show". Expected an object of type or a polymorphic subclass of this type.

我遵循here的答案。我不明白是什么原因导致了错误。欢迎任何帮助。

Answer 1

many to many relationship 之间似乎有点混淆。并使用 association object pattern (这是多对多的变体)。鉴于您配置模型的方式 Show.studio 实际上是 one to many relationship与 ShowStudio 一起使用，因此它需要 ShowStudio 对象的集合，而不是 Studio:

studio = Studio(name="asd")    
db.session.add(studio)

showstudio = ShowStudio(studio=studio)
# You've disabled save-update cascade, so you have to
# add the association object to the session manually!
db.session.add(showstudio)

s = Show(title="a", studio=[showstudio])
db.session.add(s)

db.session.commit()

对于您最初的工作尝试，您需要使用 ShowStudio 底层表作为 Show 和 Studio 之间的多对多关系。 secondary :

class Show(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    title = db.Column(db.String(200), unique=True, nullable=False)
    studio = db.relationship("Studio", secondary="show_studio",
                             back_populates="show")

class Studio(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(100), unique=True, nullable=False)
    show = db.relationship("Show", secondary="show_studio",
                           back_populates="studio")

class ShowStudio(db.Model):
    show_id = db.Column(db.Integer, db.ForeignKey(Show.id, ondelete="CASCADE"),
                        primary_key=True)
    studio_id = db.Column(db.Integer, db.ForeignKey(Studio.id, ondelete="CASCADE"),
                          primary_key=True)

由于 ShowStudio 中没有明确的 __tablename__ Flask-SQLAlchemy automatically sets it for you到show_studio。