Coincidentally, I'm having the exact same problem but only when trying to save new children. Existing ones save fine. The problem seems to be that the foreign key is not populating (a record does get saved, just with no foreign key). Is the syntax right, in that we just have to name the field or is there more needed?

I've run through the debugger and get down the SaveChildObjects() and PopulateForeignKeyField() and the value that it's trying to save is zero, rather than the parent object's primary key value.

Eric