newObjectIDForEntity in NSIncrementalStore

Question

I have started using NSIncrementalStore. When you process a fetch request, you first have to process the predicate to get your internal reference objects. Then you convert these to objectID's and then you ask the context to get the corresponding managedObjects. At least that is my interpretation of the available documentation.

let fetchedReferences : [Int] = Array(names.keys)  //names represent my backingstore
var fetchedObjectIDs : [NSManagedObjectID] = []
for reference in fetchedReferences
{
    fetchedObjectIDs.append(self.newObjectIDForEntity(request.entity, referenceObject: reference))
}
var fetchedObjects : [NSManagedObject] = []
for objectID in fetchedObjectIDs
{
    fetchedObjects.append(context.objectWithID(objectID))
}

"newObjectIDForEntity" is also used to obtain permanent objectID's (see obtainPermanentIDsForObjects)

I want to know what "newObjectIDForEntity" does. Does it make a new instance for the same object or does it each time internally create a new object? What I mean is this: if I create a new managed object and then fetch the object, I will have called "newObjectIDForEntity" twice for the same object. Does core data now think there are 1 or 2 objects?

Answer 1

Does it make a new instance for the same object or does it each time internally create a new object?

newObjectIDForEntity:referenceObject: is one of two utility methods for mapping between the store's internal representation of a managed object snapshot and an NSManagedObjectID. It's inverse is referenceObjectForObjectID:. As you might guess from the name, newObjectIDForEntity:referenceObject: returns an object considered to have a retain count of 1. newObjectIDForEntity:referenceObject: calls an internal factory method for generating an NSManagedObjectID that is unique to that reference object in this persistent store. Once that has been done, referenceObjectForObjectID: can look up that NSManagedObjectID and return the reference object it represents.

What I mean is this: if I create a new managed object and then fetch the object, I will have called "newObjectIDForEntity" twice for the same object. Does core data now think there are 1 or 2 objects?

I assume you mean an NSManagedObjectContext that is using your store creates the managed object. You can call newObjectIDForEntity:referenceObject: as many times as you want, the NSManagedObjectID instance may be different, but the data it represents is unchanged. It will know that it points to the same reference object as an earlier call with the same reference data and entity description.