Nested to-many relationships with RestKit

Question

I'm trying to map relationships for the following JSON to a CoreData backed model.

Here's the JSON:

{
    "photos": [
        {
            "id": 1,
            "thumb": "http://localhost/test_1_thumb.png",
            "image": "http://localhost/test_1.png",
            "date": "2011-07-02T06:06:16Z",
            "likes": 0,
            "user": {
                "id": 1,
                "username": "User1",
                "avatar": "http://cdn.domain.com/avatar.jpg"
            },
            "comments": [
                {
                    "date": "2011-07-02T06:06:16Z",
                    "text": "This is the only comment",
                    "id": 1,
                    "author": {
                        "username": "User1",
                        "id": 1,
                        "avatar": "http://cdn.domain.com/avatar.jpg"
                    }
                }
            ]
        }
    ]
}

My CoreData models which I've mapped with RestKit's OM2 classes.

The Photo class.

@interface Photo : NSManagedObject

@property (nonatomic, retain) NSString * thumbnailUrl;
@property (nonatomic, retain) NSString * imageUrl;
@property (nonatomic, retain) NSDate * date;
@property (nonatomic, retain) NSNumber * likes;
@property (nonatomic, retain) NSNumber * photoID;
@property (nonatomic, retain) User *owner;
@property (nonatomic, retain) NSSet *comments;

@end

@interface Photo (CoreDataGeneratedAccessors)

- (void)addCommentsObject:(Comment *)value;
- (void)removeCommentsObject:(Comment *)value;
- (void)addComments:(NSSet *)values;
- (void)removeComments:(NSSet *)values;

@end

@implementation Photo

@dynamic thumbnailUrl;
@dynamic imageUrl;
@dynamic date;
@dynamic likes;
@dynamic photoID;
@dynamic owner;
@dynamic comments;

@end

Now the part I'm not completely sure on is the relationships:

// Map relationships between entities.
[commentMapping mapKeyPath:@"author" toRelationship:@"author" withMapping:userMapping];
[photoMapping mapKeyPath:@"user" toRelationship:@"owner" withMapping:userMapping];
[photoMapping mapKeyPath:@"comments" toRelationship:@"comments" withMapping:commentMapping];

With this I'm able to access all but the comments attribute of a photo, with this error: Comments = Relationship 'comments' fault on managed object. I have a feeling this is to do with the Photo model having comments defined as an NSSet or I'm doing something wrong with the relationship mapping but I'm not sure.

Answer 1

So turns out that the problem was nothing to do with RestKit or the mapping but my understanding of how CoreData functions.

I was accessing the mapped JSON objects in my delgate as follows:

- (void)objectLoader:(RKObjectLoader*)objectLoader didLoadObjects:(NSArray*)objects {
    NSLog(@"Loaded photos: %@", objects);

    for (id object in objects) {
        Photo *photo = object;
        // Print other photo attributes...

        NSLog(@"Comments = %@", photo.comments );
    }
}

The problem was in trying to print the collection object directly. CoreData would not fire a fault on the data store. Simply replacing with a fast iteration as follows and accessing the properties allowed me to access the comments object.

for (Photo *photo in objects) {
    for (Comment *comment in photo.comments) NSLog(@"Comment = %@", comment.text);
}

Answer 2

You need something like:

[objectManager.mappingProvider setMapping:commentMapping forKeyPath:@"comments"];

Also checkout https://github.com/RestKit/RestKit/wiki/Object-mapping