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。

一张幻灯片可以用于多个引用(使用不同的组(：主键是slide_id+group_id，它必须是唯一的。

当删除指向组或幻灯片时，我希望删除所有引用。我通过在关系的backref中添加级联来实现：

# 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')
)

每当我删除引用并且没有其他引用使用所述组和/或幻灯片时，我还希望删除引用的组或幻灯片。

我曾考虑过在需要时使用@listens_for(Reference, 'after_delete')手动删除家长。在这段关系中，我也尝试过属性组合，但我真的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()

您将看到，在第一种情况下，Preference、Group和Slide被删除。因为其他Preference与Group和Slide没有相关性。但在第二种情况下，您将看到只有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>]

希望这能有所帮助。