Restkit post with core data entities and relationship

Question

I am trying to post a contents of an entity(Articles) and its relationship(Authors) with Restkit object mapping.

Articles <->> Authors An article can have many authors.

My current object mapping is as below,

    RKObjectMapping *authorMapping = [RKObjectMapping requestMapping];
    [authorMapping addAttributeMappingsFromArray:@[@"name",@"email"]];

    RKObjectMapping *articleMapping = [RKObjectMapping requestMapping];
   [articleMapping addAttributeMappingsFromArray:@[@"title",@"body",@"date"]];

   RKRelationshipMapping * rel =[RKRelationshipMapping relationshipMappingFromKeyPath:@"author" toKeyPath:@"author" withMapping:authorMapping];
   [articleMapping addPropertyMapping:rel];

    RKObjectMapping *mapping = [RKObjectMapping requestMapping];
   [mapping addPropertyMapping:[RKRelationshipMapping relationshipMappingFromKeyPath:@"singleArticle" toKeyPath:@"singleArticle" withMapping:articleMapping]];

For this mapping I was expecting a JSON output with an array of authors as below,

  {
    "singleArticle": {
        "author": [
            {
                "email": "dadsa@",
                "name": "rk"
            },
            {
                "email": "3ldll",
                "name": "rjk"
            }
        ],
        "body": "body content",
        "date": "2014-05-16T15:54:40Z",
        "title": "some title"
    }
}

But the generated JSON Output turns out with all the fields in the author class as a separate object as below,

{
    "singleArticle": {
        "author": [
            {
                "email": "dadsa@"
            },
            {
                "name": "rk"
            },
            {
                "email": "3ldll"
            },
            {
                "name": "rjk"
            }
        ],
        "body": "body content",
        "date": "2014-05-16T15:54:40Z",
        "title": "some title"
    }
}

I am not sure how what I am missing here that messes up the JSON feed as above. Any thoughts to produce a proper JSON with an array of author objects?