NSIncrementalStore: Object's persistent store is not reachable from this NSManagedObjectContext's coordinator

Question

I am trying to implement an NSIncrementalStore which will access a server over an RPC API. It needs to cache all the data locally, and so I have started by creating a NSIncrementalStore to use another core data stack which is used as the cache.

So first I set up the metadata and set up the core data stack that the cache will use. The cache SQL file is the URL passed in when the store is initialised:

    - (BOOL)loadMetadata:(NSError *__autoreleasing *)error {

NSMutableDictionary *mutableMetadata = [NSMutableDictionary dictionary];
[mutableMetadata setValue:[[NSProcessInfo processInfo] globallyUniqueString] forKey:NSStoreUUIDKey];
[mutableMetadata setValue:RTkCachedAPIStoreType forKey:NSStoreTypeKey];
//[mutableMetadata setValue:NSStringFromClass([self class]) forKey:NSStoreTypeKey];
[self setMetadata:mutableMetadata];

// Set up the Cache Stack
NSManagedObjectModel *model = [self.persistentStoreCoordinator.managedObjectModel copy];
self.cacheModel = model;

self.cachePersistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:self.cacheModel];

self.cacheContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];

NSURL *storeURL;
if ([self.URL isFileURL]) {
    storeURL = self.URL;
}

NSError *storeAddError = nil;
if (![self.cachePersistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType
                                                        configuration:nil
                                                                  URL:storeURL
                                                              options:nil
                                                                error:&storeAddError]) {
    NSLog(@"Fatal error while creating persistent store: %@", storeAddError);
    abort();
}

[self.cacheContext setPersistentStoreCoordinator:self.cachePersistentStoreCoordinator];

return YES;
}

The execute request for a simple request for an entity look like the following code. Essentially I'm just taking the request, wrapping it up as a FetchRequest and performing the request on the Cache Core data stack. I then create the objects using the returned objectID's on the calling context:

- (id)executeRequest:(NSPersistentStoreRequest *)request withContext:(NSManagedObjectContext *)context error:(NSError *__autoreleasing *)error {

if ([request requestType] == NSFetchRequestType) {
    NSFetchRequest *fetchRequest = (NSFetchRequest *)request;
    NSEntityDescription *entity = [fetchRequest entity];

    NSFetchRequest *cacheFetchRequest = [[NSFetchRequest alloc] init];
    cacheFetchRequest.entity = entity;
    __block NSArray *fetchedObjectFromCache;
    [self.cacheContext performBlockAndWait:^(){
        fetchedObjectFromCache = [self.cacheContext executeFetchRequest:cacheFetchRequest error:error];
    }];
    NSMutableArray *fetchedObjects = [NSMutableArray arrayWithCapacity:[fetchedObjectFromCache count]];
    for (NSManagedObject *anObject in fetchedObjectFromCache) {
        NSManagedObjectID *objectID = anObject.objectID;
        NSManagedObject *managedObject = [context objectWithID:objectID];
        [fetchedObjects addObject:managedObject];
    }
    return fetchedObjects;
}
}

When I try this out it throws an exception with the error

<unknown>:0: error: -[] : Object's persistent store is not reachable from this NSManagedObjectContext's coordinator

at the lineNSManagedObject *managedObject = [context objectWithID:objectID]; I can't work out why this is - this is the exact way that apple suggest this is done in the docs!

This error seems to be associated with accessing a context across threads - but I'm not doing that. Any suggestions as to what is wrong here?

Answer 1

You're using the wrong objectID.

The objectID from your cache will not match the objectID of your persistent store because your incremental store does not have the same identifier.