RestKit: Creating stubs for foreign key relationships

Question

I am struggling to map the foreign key relationships returned by my JSON API with RestKit.

Specifically, I use Loopback to generate an API with Entities like Team and User. There are two Endpoints by default that return the following JSON:

/Teams/

[
  {
    "title": "Team Title",
    "id": 1,
  }
]

/Users/

[
  {
    "name": "Alice",
    "id": 1,
    "teamId": 1,
  },
  {
    "name": "Bob",
    "id": 2,
    "teamId": 1,
  }, ...
]

Now this is a pretty simple API, right? It should be easy to map with RestKit, but even after reading all those other questions about the topic (e.g. Seeking recommendations for best RestKit/CoreData mapping and JSON structure for shallow routes, Foreign key relationship mapping with RestKit, Restkit: GET remote linked object when foreign key refers to missing local object in Core Data) I just can't figure out how to load the entire object graph into core data.

If it makes things easier, I can alter the API as well, of course. For obvious reasons, I don't want to embed the entire Team object in every User but use a foreign key instead. Also, I want to keep the Endpoints separate, mostly because this is the way Loopback generates the API by default.

So what I tried is calling both Endpoints (in an order that should not matter) with the following mapping:

Team Mapping

RKEntityMapping *teamMapping = [RKEntityMapping mappingForEntityForName:@"Team" inManagedObjectStore:objectManager.managedObjectStore];
[teamMapping addAttributeMappingsFromArray:@[ @"title", @"id" ]];
teamMapping.identificationAttributes = @[ @"id" ];
[objectManager addResponseDescriptor:[RKResponseDescriptor responseDescriptorWithMapping:teamMapping method:RKRequestMethodAny pathPattern:@"Teams" keyPath:nil statusCodes:RKStatusCodeIndexSetForClass(RKStatusCodeClassSuccessful)]];

User Mapping

RKEntityMapping *userMapping = [RKEntityMapping mappingForEntityForName:@"User" inManagedObjectStore:objectManager.managedObjectStore];
[userMapping addAttributeMappingsFromArray:@[ @"name", @"id", @"teamId" ]];
userMapping.identificationAttributes = @[ @"id" ];
[userMapping addConnectionForRelationship:@"team" connectedBy:@{ @"teamId": @"id" }];
[objectManager addResponseDescriptor:[RKResponseDescriptor responseDescriptorWithMapping:userMapping method:RKRequestMethodGET pathPattern:@"Users" keyPath:nil statusCodes:RKStatusCodeIndexSetForClass(RKStatusCodeClassSuccessful)]];

As suggested in other questions (like the ones linked above), I use an RKConnectionDescription to establish the relationship via a transient teamId property. The mapping works fine with a JSON payload that includes both Team and User objects as arrays (with a keyPath instead of a pathPattern in the response descriptor), but fails with separate Endpoints that I call independantly from each other. In that case, the object the connection relationship refers to may not have been created yet. So to solve this, a Stub Object should be created with only the id property populated in that situation.

How can I achieve this with a RestKit mapping?

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Edit: Would it help if I changed the JSON structure from "teamId": 1 to "team": { "id": 1 } and add another response descriptor with the keyPath team? Not sure if that's easy to do with Loopback. What's the best way to do this?

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Edit 2: So I added the following stub mapping:

RKEntityMapping *teamStubMapping = [RKEntityMapping mappingForEntityForName:@"Team inManagedObjectStore:objectManager.managedObjectStore];
[teamStubMapping addAttributeMappingsFromDictionary:@{@"teamId": @"id"}];
teamStubMapping.identificationAttributes = @[ @"id" ];
[objectManager addResponseDescriptor:[RKResponseDescriptor responseDescriptorWithMapping:teamStubMapping method:RKRequestMethodAny pathPattern:@"Users" keyPath:nil statusCodes:RKStatusCodeIndexSetForClass(RKStatusCodeClassSuccessful)]];

When I turn on trace logging, I don't see this mapping executed, though. What am I doing wrong? Is this mapping skipped because there is another one already matching the same pattern? When I comment out the User mapping, it does its job.

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Edit 3: So RestKit does actually use only the last response descriptor added to the object manager of those matching a given path and with equal key paths, as discussed here on GitHub. Thanks to the answer below, using a nil key path mapping solves the issue:

RKEntityMapping *teamStubMapping = [RKEntityMapping mappingForEntityForName:@"Team inManagedObjectStore:objectManager.managedObjectStore];
[teamStubMapping addPropertyMapping:[RKAttributeMapping attributeMappingFromKeyPath:nil toKeyPath:@"id"]];
teamStubMapping.identificationAttributes = @[ @"id" ];
[objectManager addResponseDescriptor:[RKResponseDescriptor responseDescriptorWithMapping:teamStubMapping method:RKRequestMethodAny pathPattern:@"Users" keyPath:@"teamId" statusCodes:RKStatusCodeIndexSetForClass(RKStatusCodeClassSuccessful)]];

Answer 1

I thought my below answer previously worked, but maybe I always snuck around the issue in the comments about exact matches of response descriptors.

So, create another mapping, but make it a nil keypath mapping and use a keypath on the response descriptor of @"teamId". This will cause RestKit to extract an array of team ids from the response and process them as object items, separate to the user objects.

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The JSON is fine. Just create a new response descriptor which uses a teamStubMapping, which is a new mapping like teamMapping but replacing id with teamId. The path pattern of this response descriptor should be @"Users".