Parse sort query via pointer

Question

I need to do a simple query sort with Parse.

I have two classes: Captain and Boat. Every Captain has a Boat point

Answer 1

It would be nice if you could use dot notation on the sort qualifiers, like:

[captainQuery orderByAscending:@"boat.name"];

Unfortunately the attribute dot notation applies only to includeKey:. So the way to do this to fetch, including the related object, then sort locally using the attribute of the related objects:

// ...
[captainQuery includeKey:@"boat"];
[query findObjectsInBackgroundWithBlock:^(NSArray *captains, NSError *error) {
    NSArray *sortedCaptains = [captains sortedArrayUsingComparator: ^(PFObject *capA, PFObject *capB) {
        return [capA[@"boat"][@"name"] compare:capB[@"boat"][@"name"]];
        // or reverse param order for descending
    }];
}];

EDIT - @Logan helpfully points out that this answer doesn't scale well if your query pages through thousands of captains. If the captain count is huge, it might be better to include a boat->captain pointer and do the following:

More roundabout-ly, you could fetch boats, orderByAscending on their name, includeKey their captains (if your boats have a back-pointer to captain), and then map over the returned (sorted) boats for their captains.

Answer 2

You can use relation query for multiple queries, the query code should looks like below:

PFObject *captain = [PFObject objectWithClassName:@"Captain"];
PFRelation *relation = [captain relationForKey:@"Boat"];
PFQuery *relQuery = [relation query];
[relQuery orderByDescending:@"name"];
// Add other query constraints

[relQuery findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {
    if (error) {
       // There was an error
    } else {
       // All the captains sorted by related boat's names
    }
}];

There are some useful links below:

http://blog.parse.com/2012/05/17/new-many-to-many/ https://parse.com/docs/ios_guide#objects-pointers/iOS

Answer 3

There is a scalable solution for this, with rather minimal overhead: Currently you have a table of boats and a table of captains, where each captain contains a pointer to a boat.

You should add a third table - let'c call it 'captains boats', which contains all the boats that are contained in a captain object. You maintain it by adding the boat when a new captain is added and by removing it from the table when a captain is deleted.

In order to sort the captains by their boats:

1. Query 'CaptainsBoats', sorted by whatever. Notice that even if this table contains a million records, you will get the first X that are relevant to you.

2. Query 'Captains', use the 'containedIn' method with the boat list from step 1: [captainQuery whereKey:"boat" containedIn:boatList].

3. Now you have only the captain objects you need (out of a possible million). Sort them locally according to whatever.