I'm preparing a Core Data mapping model using the GUI in Xcode. In the old
model, I had, say, an Animal entity. In the new model, I am replacing Animal
with two new entities, Bird and Fish.
I can distinguish Birds from Fish because birds have a beak but fish do not.
The old Animal entity had a relationship to Beak, 'beak', used for Birds only.
Aha!
I can't find any examples, but I've guessed that the "Source Fetch" is what I
need. I created two Entity Mappings, AnimalToBird and AnimalToFish. After
trying various incantations from the Core Data Migration Guide and Predicate
Programming Guide, I found that Xcode accepted my input if I set "Source Fetch"
to Default and entered one of these as "Filter Predicate":
($source.beak != nil) // Bird
($source.beak == nil) // Fish
Because I'm working on a huge mapping model, I want to reduce uncertainties.
The document "Xcode Mapping Tool for Core Data" gives only a trivial example,
and NSEntityMigrationPolicy doesn't mention filters or predicates; this feature
seems to be an "Xcode exclusive bonus".
So I'd appreciate if someone could indicate whether or not I've done this
correctly.
Jerry Krinock
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