RestKit: How to use RKMappingOperation - json

Question

I have a JSON structure retreived via web socket, that I want to apply to an existing managed object. The particular object to be modified is identified by one of the keys in the JSON. The JSON may not contain all the attributes, but I only want to update the attributes that are present in the JSON (not nullify the others).
I got some initial pointers on the RestKit IRC channel to use RKMappingOperation but am now stuck on the implementation.
First I tried this:
RKMappingOperation *mappingOperation = [[RKMappingOperation alloc] initWithSourceObject:parsedObject destinationObject:nil mapping:[MyManagedObjectClass customMapping]];
Since I don't have the instance of the object to be updated at hand, I passed nil to destinationObject, hopping the mapper would figure it out based on the mapping provided.
Alas, I get nil from mappingOperation's mappingInfo after performing the mapping (but no error).
[mappingOperation performMapping:&localError];
if (localError != nil) {
NSLog(#"%#", [mappingOperation mappingInfo]); // outputs nil
} else {
NSLog(#"error: %#", localError); // no error
}
So my hunch is that I do indeed need to get the managed object instance that I want updated and provide it to the mapping operation, but I can't figure out how. I tried using existingObjectWithID on the managed object context, passing it the ID in my JSON, but no luck. When passing that to the mapping operation i get a 'null' error.
Am I on the right track? What am I missing?
EDIT: After fiddling around some more, I realize the docs specify that you must provide a dataSource if destinationObject is set to nil. So here's what I tried next:
RKManagedObjectMappingOperationDataSource *mappingDS = [[RKManagedObjectMappingOperationDataSource alloc] initWithManagedObjectContext:[[[RKObjectManager sharedManager ] managedObjectStore] mainQueueManagedObjectContext] cache:[[[RKObjectManager sharedManager] managedObjectStore] managedObjectCache]];
mappingOperation.dataSource = mappingDS;
Embarrassingly, I had also mixed up the error conditional, so that's why I couldn't see the error on the previous attempt (without data source). Now it seems that the mapping operation performs successfully. Will report back if it's actually the case, and answer my own question :-)

Answer 1

The solution:
RKMappingOperation *mappingOperation = [[RKMappingOperation alloc] initWithSourceObject:parsedObject destinationObject:nil mapping:[MyManagedObjectClass customMapping]];
RKManagedObjectMappingOperationDataSource *mappingDS = [[RKManagedObjectMappingOperationDataSource alloc] initWithManagedObjectContext:[[[RKObjectManager sharedManager ] managedObjectStore] mainQueueManagedObjectContext] cache:[[[RKObjectManager sharedManager] managedObjectStore] managedObjectCache]];
mappingOperation.dataSource = mappingDS;
[mappingOperation performMapping:&localError];
if (localError != nil) {
NSLog(#"%#", [mappingOperation mappingInfo]);
} else {
NSLog(#"error: %#", localError);
}