On 7/18/15 5:22 AM, Martin Pengelly-Phillips wrote:
Hello,
I'm curious about the behaviour of foreign keys when modifying
collections as I want to be able to get the value before flush for
some business logic validation.
The following is a cut down version of the association list example:
|
importuuid
fromsqlalchemy importColumn,ForeignKey,Integer,String,create_engine
fromsqlalchemy.orm importSession,relationship,backref
fromsqlalchemy.ext.declarative importdeclarative_base
Base=declarative_base()
classTreeNode(Base):
__tablename__ ='tree'
id =Column(String(36),primary_key=True)
parent_id =Column(Integer,ForeignKey(id))
children =relationship(
'TreeNode',backref=backref('parent',remote_side=id)
)
def__init__(self,id=None,*args,**kwargs):
ifid isNone:
id =str(uuid.uuid4())
self.id =id
super(TreeNode,self).__init__(*args,**kwargs)
if__name__ =='__main__':
engine =create_engine('sqlite://')
Base.metadata.create_all(engine)
session =Session(engine)
node =TreeNode()
session.add(node)
printnode.id # some uuid
child_node =TreeNode()
node.children.append(child_node)
# Question: Why is parent_id not set to node.id even though it was
available?
printchild_node.parent_id
|
Note that the child_node.parent_id is None even though the referenced
value was present in the session. I know a flush will fix this, but I
want to check this value is not None for certain 'types' of node
before flush if possible.
Is there a way to configure that behaviour or should I be creating my
own collection listeners to handle this?
yes use either the AttributeEvents.append() event or
SessionEvents.before_flush().
cheers,
Martin
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