Request Mapping in RestKit - ios

Question

https://github.com/RestKit/RestKit Only introduced mapping of JSON :
{
a : "a",
b : "b"
}
Mapping with Objective-C code:
RKObjectMapping *requestMapping = [RKObjectMapping requestMapping];
[requestMapping addAttributeMappingsFromDictionary:#{#"entityId" : #"id"}];
But it do not show me how to mapping JSON Like:
{
entity: {
id: "xxxxxxxxx",
model : {
a : "a",
b : "b"
}
}
}
I did some attempt,
I create two model class like:
#interface Entity : NSObject
#property (strong, nonatomic) NSString *entityId;
#property (strong, nonatomic) NSObject *model;
#end
and
#interface Model : NSObject
#property (strong, nonatomic) NSString *a;
#property (strong, nonatomic) NSString *b;
#end
I make some RelationShipMapping Like ReponseMapping
RKObjectManager *objectManager = [RKObjectManager sharedManager];
RKObjectMapping *requestMapping = [RKObjectMapping requestMapping];
[requestMapping addAttributeMappingsFromDictionary:#{#"entityId" : #"id"}];
RKObjectMapping *modelMapping = [RKObjectMapping requestMapping];
[modelMapping addAttributeMappingsFromDictionary:#{#"a" : #"b"}];
RKRelationshipMapping *modelToEntityMapping = [RKRelationshipMapping relationshipMappingFromKeyPath:#"model" toKeyPath:#"model" withMapping:modelMapping];
[requestMapping addPropertyMapping:modelToEntityMapping];
RKRequestDescriptor *requestDescriptor = [RKRequestDescriptor requestDescriptorWithMapping:requestMapping objectClass:[Entity class] rootKeyPath:nil method:RKRequestMethodAny];
[objectManager addRequestDescriptor:requestDescriptor];
the following code I create the object
Entity *entity = [Entity new];
entity.entityId = #"/example/entityId";
Model *exampleModel = [ExampleModel new];
exampleModel.a = #"a";
exampleModel.b = #"b";
entity.model = exampleModel;
But I log the post JSON on my server:
{ id: '/example/entityId', 'model[a]': 'a', 'model[b]': 'b' }
So how to make a relationship when use requestMapping？