在 sqlalchemy 中删除所有子对象后删除父对象

Question

假设我有以下关系：

Reference(slide_id, group_id) 其中引用是幻灯片和组的关联。

一张幻灯片可用于多个引用（具有不同的组）：主键是slide_id + group_id，必须是唯一的。

我希望在删除指向的组或幻灯片时删除所有引用。 我通过在关系的 backref 中添加级联来做到这一点：

# definition of Reference model

slide = db.relationship(
    Slide,
    backref=db.backref('references', cascade='all, delete, delete-orphan')
)

group = db.relationship(
    Group,
    backref=db.backref('references', cascade='all, delete, delete-orphan')
)

我还希望删除引用的组或幻灯片，只要我删除一个引用并且没有其他引用使用该组和/或幻灯片。

我曾考虑在需要时使用@listens_for(Reference, 'after_delete') 手动删除父母。我也尝试了关系中的属性组合，但我不确定如何解决这个问题。

Answer 1

我认为@listens_for 的想法还不错。你只需要听Referencebefore_delete。下面是一个可以提供帮助的示例。

class Slide(db.Model):
    __tablename__ = 'slide'

    id = db.Column(db.Integer, primary_key=True)
    children = db.relationship('Reference')
    references = db.relationship('Reference', back_populates='slide')


class Group(db.Model):
    __tablename__ = 'group'

    id = db.Column(db.Integer, primary_key=True)
    references = db.relationship('Reference', back_populates='group')


class Reference(db.Model):
    __tablename__ = 'reference'

    id = db.Column(db.Integer, primary_key=True)
    slide_id = db.Column(db.Integer, db.ForeignKey('slide.id'))
    group_id = db.Column(db.Integer, db.ForeignKey('group.id'))
    group = db.relationship('Group', back_populates='references')
    slide = db.relationship('Slide', back_populates='references')


@event.listens_for(Reference, 'before_delete')
def delete_reference(mapper, connection, target):
    # after_flush used for consistent results
    @event.listens_for(Session, 'after_flush', once=True)
    def receive_after_flush(session, context):
        # just do here everything what you need...
        # if our preference slide is the last one
        if target.slide and not target.slide.references:
            print('add slide with id = %s to delete' % target.slide.id)
            session.delete(target.slide)
        # if our preference group is the last one
        if target.group and not target.group.references:
            session.delete(target.group)
            print('add group with id = %s to delete' % target.group.id)

让我们检查一下：

# clear tables
Reference.query.delete()
Group.query.delete()
Slide.query.delete()
# create one reference with only 1 group and only 1 slide
reference = Reference(group=Group(), slide=Slide())
db.session.add(reference)
db.session.commit()

print('reference with id %s was created. slide_id = %s, group_id = %s' % (
    reference.id, reference.slide_id, reference.group_id
))
reference = Reference.query.filter_by(id=reference.id).first()
print('add reference with id = %s to delete' % reference.id)
db.session.delete(reference)
db.session.commit()
print('amount references after delete: %s' % Reference.query.all())


# create 2 references but with the same group and slide
group = Group()
slide = Slide()
reference = Reference(group=group, slide=slide)
reference2 = Reference(group=group, slide=slide)
db.session.add(reference)
db.session.add(reference2)
db.session.commit()
print('reference with id %s was created. slide_id = %s, group_id = %s' % (
    reference.id, reference.slide_id, reference.group_id
))
print('reference2 with id %s was created. slide_id = %s, group_id = %s' % (
    reference2.id, reference2.slide_id, reference2.group_id
))

reference = Reference.query.filter_by(id=reference.id).first()
print('add reference with id = %s to delete' % reference.id)
db.session.delete(reference)
print('amount references after delete: %s' % Reference.query.all())
db.session.commit()

您会看到在第一种情况下Preference、Group 和Slide 已被删除。因为其他Preference 与Group 和Slide 无关。但在第二种情况下，您会看到只有 Preference 被删除：

reference with id 76 was created. slide_id = 65, group_id = 65
add reference with id = 76 to delete
add slide with id = 65 to delete
add group with id = 65 to delete
amount references after delete: []
reference with id 77 was created. slide_id = 66, group_id = 66
reference2 with id 78 was created. slide_id = 66, group_id = 66
add reference with id = 77 to delete
# with slide_id = 66, group_id = 66
amount references after delete: [<Reference 78>]

希望这会有所帮助。