What's the difference between -objectRegisteredForID: and -existingObjectWithID:error:?

Question

What's the difference between getting an managed object with

- (NSManagedObject *)objectRegisteredForID:(NSManagedObjectID *)objectID

and

- (NSManagedObject *)existingObjectWithID:(NSManagedObjectID *)objectID error:(NSError **)error

What are "registered" objects? What's the difference between "registered" objects and "unregistered" objects?

Answer 1

What are "registered" objects?

Judging from the results I've gotten using these methods, a registered object is one that has been fetched into the MOC. If an object exists in the persistent store but has not been fetched, feeding its objectID to the objectRegisteredForID method will return nil.

How could you even have its objectID if it had not been fetched? Well, I visited this question when implementing a Revert routine. I dumped any unsaved changes by replacing the database with an older copy, cleared the context and then reaccessed it. But I wanted to be able to restore the user's selection of objects to the cache of a table. So, before doing the reversion, I stashed the objectIDs that the user had selected in an array. Then, after the reversion, I rebuilt the table cache using the stashed objectIDs.

When I called objectRegisteredForID using these stashed objectIDs, it always returned nil. (But if I tested this before getting a fresh context, it would return the corresponding object -- which at that point was a fetched, loaded object. Hence my inference as to the meaning of "registered.")

When I called objectWithID using these stashed objectIDs, everything was fine unless the object had been deleted post the last save, in which case it would no longer exist in the database and the invalid but non-nil return would cause exceptions later.

So I used existingObjectWithID:error. If the object still existed, it would be returned. If it no longer existed, the return would be nil and the error's localizedDescription would be "Attempt to access an object not found in store."

Answer 2

Years after the fact:

As Wienke suspects, registered objects are those already in memory for that context. So objectRegisteredForID: will return an object only if somebody else has previously obtained that object.

objectWithID: will return an object if it currently exists in the persistent store.

So the really important distinction is:

objectWithID may go to the persistent store.

Note the corollary: objectWithID may have to perform a fetch. That means locking the store. So:

• if the store is already locked by somebody else, objectWithID may block, whereas objectRegisteredForID will never block; and
• supposing you had an array of 30 object IDs and you performed objectWithID for each, you'd potentially do 30 separate trips to the store — it'd be much faster to check whether the objects are already registered and then, if any aren't, use an NSFetchRequest to get the rest. Notice that a self in X query can accept an array or set of object IDs to return actual objects even though that wouldn't technically exactly match the normal Objective-C meaning of 'self'.

Falling back on NSFetchRequest is also generally preferable if you have any relationship paths you're going to need prefetched. So there's potentially quite a lot of performance to be gained.

Answer 3

To your first question:

objectRegisteredForID: is the quick & easy way to get the object -- it either returns your object or it returns nil letting you know that it could not. You use this when you either already know why the result might be nil or you don't care.

existingObjectWithID:error: is similar, in that it returns either your object or nil but, in addition, if you pass an error parameter, it will tell you WHY it returned nil. You may want to do this if you plan to do any sort of error reporting (a log message or alert), or error handling (perhaps you plan to take different actions, depending on which kind of error is returned.

EDIT: In addition (per docs), if there is not a managed object with the given ID already registered in the context, the corresponding object is faulted into the context.

I suggest that you break out the "what are registered objects?" portion of your question to a separate question in order to facilitate both getting a better answer (based on the subject line matching the question) and to help future spelunkers who may come looking for an answer.