Persisting relationship with local user object during a Restkit update

Question

I am working on an events app that syncs "Events" with a JSON API using Restkit. The mapping looks roughly like this.

var eventsMapping = RKEntityMapping(forEntityForName: "Event", inManagedObjectStore: managedObjectStore)
        eventsMapping.identificationAttributes = ["eventID", "name", "eventDescription"]
        eventsMapping.addAttributeMappingsFromDictionary([
            "id":"eventID",
            "title":"name",
            "description":"eventDescription",
            (more mapping attributes here, etc...)
            ])

The events are displayed using a NSFetchedResultsController. Locally, there is a 'User' NSManagedObject that is created...however this object is just local to the app and is not synced to the server via Restkit. The "User" has a one-to-many relationship with Events, with the purpose that a user can "save" events that they are attending. (Note: Right now, both sides of the Core Data relationship delete rules is set to "no action.") The save is roughly done this way.

var managedObjectContext = RKManagedObjectStore.defaultStore().mainQueueManagedObjectContext
currentUser.mutableSetValueForKey("events").addObject(event)
managedObjectContext?.saveToPersistentStore(&error)

So far everything works great, selected events are stored and saved successfully with the user and persists through app relaunches as expected. However, there is one scenario that causes an event to be removed from the user and that is when an update is made to that particular event on the server. When Restkit detects this and updates the event, according to breakpoints I placed in the NSFetchedResultsController didChangeObject, apparently Restkit and/or Core Data is actually deleting the event and then inserting it back with the updates. This is transparent and just fine in most cases, but in this case I'm thinking the initial delete is what is breaking off the event from the user.

Of course, the eventsMapping above doesn't reference any relationship to a user in any way, so that could be another reason why the relationship is broken off. I have been reading more about Restkit relationships and I have used relationship / property mappings in Restkit before successfully to relate objects, but in that scenario both objects existed on the API. In this case, the User here isn't a part of the API at all, only local as I explained. So should I still be using a Restkit relationship mapping? Or perhaps I should be trying to accomplish all of the above via another way?

Shannon Shannon · Accepted Answer · 2015-08-27T21:26:21

I figured out the answer, I made a goof in the code above. On the identification attributes for the mapping, I had three different properties in there, when I only should have used the one with the primary key that would never change (eventID).

eventsMapping.identificationAttributes = ["eventID"]

What apparently was happening was because I specified the title/name as an identification attribute, whenever that title changed on the server Restkit would identify it as a new/different object and delete the "old" object and insert the "new" one. When I changed it only to specify the primary key, it triggered an update instead and my relationship with the user persisted.

One more note for others that threw me at first: some older info I came across that helped me solve this said to use a primaryKeyAttribute on the mapping. This is apparently dated information: use the identificationAttributes instead.

Persisting relationship with local user object during a Restkit update

1 Answers