On 05/26/2016 11:58 AM, Eric So wrote:
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:
|
classParent(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")
defvalidate_children(self,key,child):
new_quantity =self.current_children_quantity +child.some_quantity
ifself.some_quantity <new_quantity:
raiseException
returnchild
classChild(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")
defincrement_quantity(target,value,initiator):
target.current_children_quantity +=value.some_quantity
so yes the append() fires off before the append actually happens.
The two options here are either to write a custom collection class (e.g.
list subclass) that interacts with the parent if you need this value to
be always correct, or more easily you can set /validate the number just
once using a flush-time event, such as the before_flush() event handler,
or before_insert() or before_update().
|
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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