RestKit 0.20.2 generating incorrect JSON output for sending child object collection in post/put operations - restkit-0.20

Question

I have 4 NSManagedObjects i.e. Item, Listing, PaymentMethod and HandlingTime such that Item has 1-1 relation with Listing which has 1-n relation with PaymentMethod and Listing has 1-1 relation with HandlingTIme. When I try to post/put an item, following mapping works for Item-Listing and Listing-HandlingTime relationship for making item post/put server operations but does not generate correct JSON for Listing-PaymentMethods.
NSDictionary *itemRKRequestMapping = #{
#"id" : #"eid",
#"title" : #"title",
#"listing.id" : #"listing.eid",
#"listing.title" : #"listing.title",
#"listing.item.id" : #"listing.item.eid",
#"listing.handlingTime.id" : #"listing.handlingTime.eid",
#"listing.handlingTime.title" : #"listing.handlingTime.title",
#"listing.paymentMethods.id" : #"listing.paymentMethods.eid",
#"listing.paymentMethods.title" : #"listing.paymentMethods.title"
};
It produces the following JSON for RKRequest operation when calling
[[RKObjectManager sharedManager] putObject:self.item path:#"items/1" parameters:nil success:^(RKObjectRequestOperation *operation, RKMappingResult *mappingResult) {
NSLog(#"Success"); } failure:^(RKObjectRequestOperation *operation, NSError *error) {
NSLog(#"Failure saving item: %#", error.localizedDescription);
}];
In correct JSON for paymentMethods.
{"id":1, "title":"item1", "listing":{"id":1,"item":{"id":1},"handlingTime":{"id":1, "title":"20 days"},"paymentMethods":{"id" : [1,2],"title":["VISA","MASTER"]} } }
Following is the desired correct JSON output for paymentMethods.
{"id":1, "title":"item1", "listing":{"id":1,"item":{"id":1},"handlingTime":{"id":1, "title":"20 days"},"paymentMethods": [ { "id":1, "title": "VISA"}, { "id":2, "title": "MASTER"} ] } }
GET operation works fine for this object model as thats setup using the relationshipMappingFromKeyPath for item- listing relation as shown below.
[itemMapping addPropertyMapping:[RKRelationshipMapping relationshipMappingFromKeyPath:#"listing" toKeyPath:#"listing" withMapping:listingMapping]];
[listingMapping addPropertyMapping:[RKRelationshipMapping relationshipMappingFromKeyPath:#"handlingTime" toKeyPath:#"handlingTime" withMapping:handlingTimeMapping]];
[listingMapping addPropertyMapping:[RKRelationshipMapping relationshipMappingFromKeyPath:#"paymentMethods" toKeyPath:#"paymentMethods" withMapping:paymentMethodMapping]];
Can any one please point out the mistake in the mappings for RKRequest operations above?

Answer 1

Your problem is that the relationship contains only a single item so RestKit doesn't know that it should be mapped into an array. To teach RestKit, you need to separate the mapping for the appropriate key, explicitly create an attribute mapping for it, set the mapping to forceCollectionMapping and then add it to your itemRKRequestMapping.
forceCollectionMapping docs are here.