Note that in the above code, "data" represents the JSON that you want to map. "mappingsDictionary" represents a dictionary of source key path to the mapping that should be used for that data. Most of the time you'll just have [NSNull(): RKManagedObjectMapping] here, but an example of when you may need something else is if you, for example, wrap your API's response in an envelope:
Note that RKMapperOperation will not automatically save the NSManagedObjectContext on completion. You'll need to save after the operation is complete (you can use RKMapperOperation's delegate to receive events about the progress.
Final note - best practices suggest you'll want to use a child NSManagedObjectContext for each mapping operation. This ensures that you can always recover back to a consistent state in the case of a mapping failure (simply discard the temporary context). Simply call [mappingContext save]; on the mapping context to push it up to its parent store, and periodically flush the parent store out to disk per usual core data semantics.
Hopefully this helps.