This only affects the property mapping in your model, so if you say map the property `foo` to the JSON key `bar`, this will only make sure that your class actually has a property `foo`. If your server's response actually contains `bar` or a different set of keys entirely is a different matter.

The `if (error != NULL)` is needed to make sure that we're not dereferencing a NULL pointer when assigning the NSError later.

Also check out #325 where we're changing this to an exception.