Gather all PFRelation under the same PFQuery

Question

I'm using Parse as my backend and I'm currently looking for a better way for my situation.
I'll use Objective-C in order to demonstrate but I didn't tag it as Objective-C because it's not language related.
My situation is as follows: I have a class let's call it X. that has a key let's call it Y which is PFRelation of type Z (Y<Z>). I need to get a list of Z. like to query for Z (Z is all I'm interested in). Moreover, if possible, I want to be able to limit the output of Z like PFQuery does.
To sum up: I need a PFQuery that will be able to do all the abilities of PFQuery e.g. findObjectsInBackgroundWithBlock: and countObjectsInBackgroundWithBlock that the object I will be operating in, in the block will be of NSArray of type Z.

My current solution: I thought of querying for X where exists Y (so I won't get empty objects of X. and just for every X, load all Y's and go through all arrays of Z.

I really don't think my solution is the best, hoping for someone that can point me at another solution to fit my situation.

I'm having problems grasping what it is you're after. Could you try explaining with a more mundane example objects (like cars, parts, books, authors, albums, artists etc) instead of XYZ? Maybe it's just too early in the morning for me, but... oh well :-) — Marius Waldal
Let's say X is a car workshop and Y is a PFRelation of cars and Z is the type Car. I need a query to go through so I'll have only cars that match my query on the workshops. I think this should do... Once again early in the morning. Say if it's not good enough — user2558461
Sooo, you want a list of cars, but only cars that... what? Only cars from a specific list of workshops? If the workshop meets a criteria, give me the cars they have? Can you provide some code you are already trying, or maybe some pseudo code? Or just provide the exact use case :-) — Marius Waldal
I want the list of cars that doesn't have any criteria. Its parent class, workshops meet a criteria. Like all cars that their workshop is in the usa. I don't have a psuedo... Because I'm seriously out of luck with how to make it efficient — user2558461
If you still don't understand, please tell me , I'll update my question — user2558461

Marius Waldal Marius Waldal · Accepted Answer · 2014-02-13T09:20:09

I think might be what you are looking for:

PFQuery *wsQuery = [PFQuery queryWithClassName:@"Workshop"];
[wsQuery whereKey:@"country" equalTo:@"USA"];  // or whatever your criterias
NSArray *workshops = [wsQuery findObjects];

for (PFObject *workshop in workshops) {
    PFQuery *carsQuery = [workshop relationforKey:@"cars"].query;
    [carsQuery setLimit:5];
    // Add whatever constraints you want here
    NSArray *wsCars = [carsQuery findObjects];
    for (PFObject *car in wsCars) {
        NSLog(@"Car make: %@", [car objectForKey:@"make"]);
    }
}

This will first get a list of all workshops, and then for each workshop will get a list of all the cars belonging to that workshop, and print the make (brand) for each car.

UPDATE

New suggestion based on comments:

PFQuery *wsQuery = [PFQuery queryWithClassName:@"Workshop"];
[wsQuery whereKey:@"country" equalTo:@"USA"];

PFQuery *carQuery = [PFQuery queryWithClassName:@"Car"];
[carQuery whereKey:@"workshop" matchesQuery:wsQuery];
[carQuery setLimit:5];
[carQuery findObjectsInBackgroundWithBlock:^(NSArray *cars, NSError *error) {
    // cars contains only cars with workshops located in USA 
}];

Gather all PFRelation under the same PFQuery

1 Answers