Why is a PFRelation saved by saving the PFUser object?

Question

so I am having trouble understanding how a PFRelation saves, I have this code:

PFUser *user = [PFUser currentUser];
PFRelation *relation = [user relationForKey:@"likes"];
[relation addObject:post];
[user saveInBackground];

Why is it that updating the user ie [user saveInBackground] updates the PFRelation field "likes" for that user? Does this use a single API call or does [relation addObject: post]; also require an API call?

Any help would be greatly appreciated.

Thanks

Answer 1

In your case 1 API request is used under circumstances.

Take a look at PFObject [straight from Parse.com]:

// Create the post
PFObject *myPost = [PFObject objectWithClassName:@"Post"];
myPost[@"title"] = @"I'm Hungry";
myPost[@"content"] = @"Where should we go for lunch?";

// Create the comment
PFObject *myComment = [PFObject objectWithClassName:@"Comment"];
myComment[@"content"] = @"Let's do Sushirrito.";

// Add a relation between the Post and Comment
myComment[@"parent"] = myPost;

Here you are setting attributes or properties to the PFObject but nothing ever happens until you save it, you can do anything to the object like change it, update it, doesn't matter, but backend wise, it won't update unless you tell it to which is where save comes in play :

[myComment saveInBackground];

In short, you can add relations, pointers and numerous parameters all day long, but nothing happens until you tell it to happen : [saveInBackground];

Because you made it a direct correlation to user it saves it to that user because you told it to. Because you specified a relation to the user, once you save the user properties the relation will also be saved. However, this doesn't create more API requests.