Fetching data from two classes in a single PFQuery

Question

Imagine the hypothetical case where I have a Car class that contains attributes like color, brand, etc. A car can have zero or more owners. The Owners class has first name, last name, etc attributes. When an owner is created, I set its "carID" attribute equal to a car's objectID. Therefore you can determine the number of owners a car has had by querying the Owners table where the carID equals a certain car ID.

Now I would like to perform a single fetch request that will return all attributes of a car, plus a new attribute that's not a column in the Car class. I want to get an array containing the owners objects associated with the car. The reason I want that info in a single request is because I am fetching multiple cars at once, so I don't want to have to make a request to get the owners for every single car that is returned from that fetch request.

This is my current query:

PFQuery *query = [PFQuery queryWithClassName:@"Car"];
[query whereKey:"color" equalTo:"red"];
[query findObjectsInBackgroundWithBlock:^(NSArray *cars, NSError *error) {
    //need to know which owners each car has had so I can access attributes of each owner here
    //without making a fetch request for every car in the "cars" array
}];

How would you modify the query in order to get the desired data in a single fetch?

Answer 1

Have your car representation include an attribute called owners which is an array of pointers to the owner class. With that, you can fetch owners eagerly as part of a car query using includeKey:...

PFQuery *query = [PFQuery queryWithClassName:@"Car"];
[query includeKey:@"owners"];
[query findObjectsInBackgroundWithBlock:^(NSArray *cars, NSError *error) {
    for (PFObject *car in cars) {
        for (PFObject *owner in [cars objectForKey:@"owners"]) {
           // here we'll enumerate all of the owners for all of the cars
        }
    }
}];

And you can get cars owned by a given owner with whereKey:equalTo:, which works the same for singular and array columns...

PFObject *owner = // some owner
PFQuery *query = [PFQuery queryWithClassName:@"Car"];
[query whereKey:@"owners" equalTo:owner];
[query includeKey:@"owners"];
// etc.