example.json

{
  "user": {
    "id": 999,
    "name": "Joe Bloggs"
  },
  "stream": [
    {
      "link": {
      "url": "http://www.example.com",
      "id": 123
      }
    },
    {
      "status": {
      "text": "Hellow",
      "id": 123
      }
    },
    {
      "photo": {
      "url": "http://www.example.com/photo.jpg",
      "id": 123
      }
    }
  ]
}

mapping.m

@implementation SomeClassRepresentingJustThisResponse

// has properties for user, streamItems;

+ (RKObjectMapping *)objectMapping
{
  RKObjectMapping *mapping = [RKObjectMapping mappingForClass:self];
  
  // this one works
  RKRelationshipMapping *eventMapping = [RKRelationshipMapping relationshipMappingFromKeyPath:@"user" toKeyPath:@"user" withMapping:[User objectMapping]];
  
  [mapping addPropertyMapping:eventMapping];

  /* one mapping for each type of supported stream item */
  
  NSDictionary *streamMappings = @{
    @"image": [Image objectMapping],
    @"link": [Link objectMapping],
    @"status": [Status objectMapping]
  };
  
  RKDynamicMapping *dynamicPrerollItemMapping = [[RKDynamicMapping alloc] init];
  
  [dynamicPrerollItemMapping setObjectMappingForDataBlock:^RKObjectMapping *(NSDictionary *data) {
    NSString *type = [[data allKeys] objectAtIndex:0];
    return [prerollMappings objectForKey:type];
  }];
  
  // this one gives me a collection of the right objects, but no mapped attributes
  RKRelationshipMapping *prerollItemsMapping = [RKRelationshipMapping relationshipMappingFromKeyPath:@"stream" toKeyPath:@"streamItems" withMapping:dynamicPrerollItemMapping];
  
  [mapping addPropertyMapping:prerollItemsMapping];
  
  return mapping;
}

@end