Right way to serialize nested collection with RestKit - ios

Question

I'm using RestKit to POST my Album class with his nested collection o songs (Song class)
I cannot find the right way to serialize the NSArray containing album songs: it serialize the whole collection into a kind of collection of NSDictionary.
I'm using v 0.10.0 and using NSObjects not CoreData
Here is my json structure:
{"album":
{"id":12,
"source":"apple",
"collection_name":"The Platinum Collection",
"artist_name":"Queen",
"collection_id":"28007467",
"genre_id":5,
"thumb":"mythumburl.jpg",
"creation_user_id":1,
"songs":[
{"id":16,
"track_name":"Bohemian Rhapsody",
"track_time_millis":353933,
"disc_number":1,
"track_number":1,
"track_time":"00:05:53"},
...
]
}
This is my actual mapping:
RKObjectMapping *songMapping = [RKObjectMapping mappingForClass:[Song class]];
[songMapping mapKeyPath:#"id" toAttribute:#"code"];
[songMapping mapKeyPath:#"source" toAttribute:#"source"];
[songMapping mapKeyPath:#"album_id" toAttribute:#"album_id"];
[songMapping mapKeyPath:#"track_name" toAttribute:#"track_name"];
[songMapping mapKeyPath:#"track_time_millis" toAttribute:#"track_time_millis"];
[songMapping mapKeyPath:#"track_time" toAttribute:#"track_time"];
[songMapping mapKeyPath:#"disc_number" toAttribute:#"disc_number"];
[songMapping mapKeyPath:#"track_number" toAttribute:#"track_number"];
[songMapping mapKeyPath:#"preview_url" toAttribute:#"preview_url"];
[objectManager.mappingProvider setObjectMapping:songMapping forKeyPath:#"song"];
[objectManager.mappingProvider setSerializationMapping:[songMapping inverseMapping] forClass:[Song class]];
RKObjectMapping *albumMapping = [RKObjectMapping mappingForClass:[Album class]];
...
[albumMapping hasMany:#"songs" withMapping:songMapping];
// Setup our object mappings
[objectManager.mappingProvider setObjectMapping:albumMapping forKeyPath:#"album"];
[objectManager.mappingProvider setSerializationMapping:[albumMapping inverseMapping] forClass:[Album class]];