stackmob 1 to-many relationship architecture

Question

I'm searching for some architectural guidance for a 1 to-many relationship for an ios app with a backend that runs on stackmob (baas).

In my project I want to implement a facebook like stream of posts. Users will be allowed to write comments to the posts.

1 post -> n comments

When I create the view (iOS UITableview) of posts I want to display only the number of comments. The actual comments will be displayed in a subview. What is the best way to implement this? How should I query such related objects?

If I first fetch all the posts and then for each post I have to query the number of comments I'm afraid to run into a performance issues :-(

What is your proposal about querying 1 to-many relations without producing to much http overhead...

I found this post where I at least learned that Fetching on an attribute of a relationship (task where project.user == X) is not supported by StackMob:

NSPredicate traversing relationship (StackMob)

Thanks for your support!

Cheers,

Jan

I depends. Will you be using CoreData? – LuisCien Jun 08 '13 at 19:10 — LuisCien, Jun 08 '13 at 19:10
Yes I will use CoreData via stackmobs ios sdk – Jan Wilhelm Jun 09 '13 at 20:07 — Jan Wilhelm, Jun 09 '13 at 20:07
Alright, I'll post an answer soon. – LuisCien Jun 10 '13 at 03:33 — LuisCien, Jun 10 '13 at 03:33

score 0 · Answer 1 · answered Jun 11 '13 at 03:03

I'm assuming you've read this tutorial.

CoreData uses a mechanism known as "faulting" to reduce the application's memory usage. From Apple's docs:

"A fault is a placeholder object that represents a managed object that has not yet been fully realized, or a collection object that represents a relationship"

What this means is that when you read an Entity from CoreData it does not reads the entire object graph just to populate that one Entity.

In your case, if you read a "Post" entity it won't read all the comments related to that post. This is true even when you're using StackMob as the DataStore.

I made a very small project just to test this and this is what I got. By using the sniffer Charles I was able to see the request/response of the fetch. One-to-many relationship

Here is the query I used:

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Post" inManagedObjectContext:self.managedObjectContext];
[fetchRequest setEntity:entity];

[self.managedObjectContext executeFetchRequest:fetchRequest onSuccess:^(NSArray *results) {

    for (NSManagedObject *managedObject in results) {
        NSArray *comments = [managedObject valueForKey:@"comments"];
        NSLog(@"Post comments: %d", comments.count);
    }

} onFailure:^(NSError *error) {
    NSLog(@"Error fetching: %@", error);
}];

And here is the response I got (from using Charles)

[
  {
    "post_id": "85FC22A1-92ED-484B-9B52-78CBFE464E5D",
    "text": "Post1",
    "lastmoddate": 1370916620273,
    "createddate": 1370915371016,
    "comments": [
      "5F421C86-F7E7-4BAD-AB21-F056EB7D9451",
      "C1CD96E2-856A-43F0-A71D-09CF14732630",
      "BAD9F652-C181-4AA1-8717-05DD5B2CA54E",
      "AC7D869B-A2BD-4181-8DCF-21C8E127540F",
      "2D1A1C80-9A80-48AE-819D-675851EA17D6"
    ]
  },
  {
    "post_id": "8A84BE1B-6ECE-4C49-B706-7981490C5C2E",
    "text": "Post2",
    "lastmoddate": 1370916678114,
    "createddate": 1370915379347,
    "comments": [
      "3C001706-2D91-4183-9A07-33D77D5AB307",
      "E7DC3A89-2C3D-4510-83E5-BE13A9E626CF",
      "2874B59C-781B-4605-97C7-E4EF7965AF4E"
    ]
  },
  {
    "post_id": "13E60590-E4E7-4012-A3E2-2F0339498D94",
    "text": "Post3",
    "lastmoddate": 1370916750434,
    "createddate": 1370915398649,
    "comments": [
      "AEA3E5E3-E32E-4DAA-A649-6D8DE02FB9B2",
      "DFCBD7E2-9360-4221-99DB-1DE2EE5590CE",
      "484E6832-3873-4655-A6C1-0303A42127D9",
      "B2316F8B-0AAF-451F-A91C-9E8B4657B1A5"
    ]
  }
]

As you can see from the response, all of the "comments" arrays have the 'Id' of the comment stored in StackMob, but it doesn't have the actual comment.

Hope this helps!

stackmob 1 to-many relationship architecture

1 Answers1