iPhone RestKit is it possible to convert a Core Data object graph into JSON?

Question

I'm working with a RestKit framework for one of my projects. It includes an example of how to convert JSON objects into a core data graph using object mapping. I'm wandering if the reverse is possible - can a Core Data entity using RestKit be converted back to a JSON representation?

I found RKObjectSerializer class, but I can't seem to make it work - the serialized object that I get is nil

-(void)doCoreDataToJSONConversion
{
    Article* article = [_articles objectAtIndex:0];
    RKManagedObjectMapping* articleMapping = [RKManagedObjectMapping mappingForClass:[Article class]];
    NSAssert(articleMapping!=nil,@"article mapping is nil!");
    NSLog(@"%@",[article description]);

    RKObjectSerializer* serializer =[RKObjectSerializer serializerWithObject:article mapping:articleMapping];

    NSError* error = nil;
    NSMutableDictionary* serializedObject = [serializer serializedObject:&error];

    if(error!=nil)
    {
        NSLog(@"!!!!! Error: %@",[error localizedDescription]);
    }

    //prints nil
    NSLog(@"Serialized Object: %@", [serializedObject description]);
}

Thank you for your input!

Answer 1

Can't tell you about the RestKit exactly, but you can easily convert all your Core Data graph to JSON and back if you just take a look at this absoluetly brilliant answer by Marcus Zarra

JSON and Core Data on the iPhone

Answer 2

I ended up using these methods to convert core data objects into JSON. To deserialize these objects I need to define a different class of mapping: RKManagedObjectMapping, which is slightly different from the regular object mapping.

-(void)setupObjectSerializationMapping
{
    RKObjectManager *objectManager = [RKObjectManager sharedManager ] ;
    RKObjectMapping *mapping = [RKObjectMapping mappingForClass:[Article class]];

    [mapping mapAttributes:@"articleID", @"title", @"body", nil];

    //********************************    

    RKObjectMapping *imageMapping = [RKObjectMapping mappingForClass:[EventImage class]];

    [imageMapping mapAttributes:@"createDate", @"localFilePath", nil];
    [objectManager.mappingProvider addObjectMapping:imageMapping];
    [objectManager.mappingProvider setSerializationMapping:[imageMapping inverseMapping] forClass:[EventImage class]];
    [objectManager.mappingProvider setMapping:imageMapping forKeyPath:@"eventImages"];

    //********************************    

    RKObjectMapping *eventMapping = [RKObjectMapping mappingForClass:[Event class]];

    [eventMapping mapAttributes:@"createDate", @"note", nil];
    [eventMapping mapRelationship:@"eventImages" withMapping:imageMapping];

    [objectManager.mappingProvider addObjectMapping:eventMapping];
    [objectManager.mappingProvider setSerializationMapping:[eventMapping inverseMapping] forClass:[Event class]];
    [objectManager.mappingProvider setMapping:eventMapping forKeyPath:@"events"];

    //******************************** 
    //setup App user mapping
    RKObjectMapping *userMapping = [RKObjectMapping mappingForClass:[AppUserSubclass class]];

    [userMapping mapAttributes:@"userID",@"firstName", @"lastName",@"localDataFilepath", nil];
    [userMapping mapRelationship:@"events" withMapping:eventMapping];
    [objectManager.mappingProvider addObjectMapping:userMapping];
    [objectManager.mappingProvider setSerializationMapping:[userMapping inverseMapping] forClass:[AppUserSubclass class]];
    [objectManager.mappingProvider setMapping:userMapping forKeyPath:@"appUser"];



}

-(NSString*)generateLocalJSONData
{

    NSString* folderPath = [self dataFolderPath];
    // remember what the file was called. This will not include the documents directory and will be the same on all machines.
    NSString* localFileName = [self fileName];

    NSString* dataFile = [self fullDocumentsFilePath];
    self.localDataFilepath = dataFile;

    NSAssert(self.localDataFilepath!=nil,@"failed to save local data path");
    //********************************


    NSError* error = nil;

    //app user subclass is the same as AppUser, but is used to differentiate between file based mapping and core data mapping
    RKObjectMapping *serMap = [[[RKObjectManager sharedManager] mappingProvider] serializationMappingForClass:[AppUserSubclass class]];
    NSDictionary *d = [[RKObjectSerializer serializerWithObject:self mapping:serMap] serializedObject:&error];


    if(error!=nil)
    {
        NSLog(@"!!!!! Error: %@",[error localizedDescription]);
    }


    NSString* dataContents = [d JSONString];
    BOOL success =  [dataContents writeToFile:dataFile atomically:YES encoding:NSUTF8StringEncoding error:nil];
    if(!success)
    {
        NSLog(@"Error writing to data file!");
    }
    [[AppUser managedObjectContext] save:nil];

    return dataFile;

}