Background:
I have a two model classes where one is a parent of the other via a one to
many relationship. The parent has a collection of the children with a
backref:
class Parent(base):
__tablename__ = "parent_table"
parent_key = Column(String, primary_key=True)
some_quantity = Column(Integer)
current_children_quantity = Column(Integer)
children = relationship("Parent", backref="parent_table")
@validates("children")
def validate_children(self, key, child):
new_quantity = self.current_children_quantity + child.some_quantity
if self.some_quantity < new_quantity:
raise Exception
return child
class Child(base):
__tablename__ = "child_table"
child_key = Column(String, primary_key=True)
some_quantity = Column(Integer)
parent_key = Column(String, ForeignKey("parent_table.parent_key"))
When I add a child to the parent collection, I have to make sure that the
sum of the "some_quantity" fields of all children on the same parent do not
exceed the
some_quantity field of the parent. The field current_children_quantity
keeps track of the sum of the children. In order to automatically do this,
I set up an
event listener:
@event.listens_for(Parent.children, "append")
def increment_quantity(target, value, initiator):
target.current_children_quantity += value.some_quantity
The problem is that the "append" event fires before the validator. This
results in the parent's current_children_quantity being updated
before the child append can be validated. Because of this, in the case
where twice the child's some_quantity added to current_children_quantity
exceeds the parent's some_quantity, the append doesn't happen, even though
it should.
My (ugly) solution is to put the validation into the "append" event
listener but there must be a better way! Can I control the order of events?
Maybe I'm not using all the tools at my disposal?
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