Python 在sqlalchemy中删除所有子对象后删除父对象
假设我有以下关系:Python 在sqlalchemy中删除所有子对象后删除父对象,python,orm,sqlalchemy,associations,relationship,Python,Orm,Sqlalchemy,Associations,Relationship,假设我有以下关系: Reference(幻灯片id,组id)其中Reference是幻灯片和组的关联 一张幻灯片可用于多个参考资料(不同的组):主键是slide\u id+group\u id,必须是唯一的 我想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中添加级联来实现: # definition of Reference model slide = db.relationship( Slide, backref=db.backref('referen
Reference(幻灯片id,组id)
其中Reference是幻灯片和组的关联
一张幻灯片可用于多个参考资料(不同的组):主键是slide\u id
+group\u id
,必须是唯一的
我想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')
)
我还希望删除引用的组或幻灯片,只要我删除了引用,并且没有其他引用使用了所述组和/或幻灯片
我考虑过使用@listens\u for(Reference,'after\u delete')
在需要时手动删除家长。我也尝试过在关系中使用属性组合,但我不确定如何解决这个问题。我认为使用属性组合的想法不错。您只需收听参考
。下面是一个有帮助的例子
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,首选项
,组
和幻灯片
被删除。因为其他首选项
与组
和幻灯片
无关。但在第二种情况下,您将看到只有首选项被删除:
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>]
已创建id为76的引用。幻灯片id=65,组id=65
添加id=76的引用以删除
添加id=65的幻灯片以删除
添加id=65的组以删除
删除后的金额参考:[]
已创建id为77的引用。幻灯片id=66,组id=66
已创建id为78的引用2。幻灯片id=66,组id=66
添加id=77的引用以删除
#幻灯片id=66时,组id=66
删除后的金额参考:[]
希望这有帮助