Adding value to NSObject key- Expression is not assignable

Question

I’m trying to add a value I get back from an API call to an object in Objective-C. I get an error in Xcode saying “Expression is not assignable” and don’t understand why. In my carObj I have an object called warrantyPlan with a nil value and I’m trying to set the value for warrantyPlan. What am I doing wrong in this method?

NSArray *carUUIDs = [carData valueForKeyPath:@"uuid"];
NSString *ownerUUID = ownerRecord[@"uuid"];
if (ownerID) {
    NSManagedObjectContext *context = [DataCenter viewContext];
    NSObject *carObj = [context objectsForEntityName:[CarObject entityName] matchingUUIDs:carUUIDs];
    
    for (id carID in carUUIDs){
        [WarrantyPlansService getWarrantyPlansForCarWithCarID:carID ownerID:ownerUUID completion:^(NSArray* response, NSError* error){
            //attach the response to the carData
            NSLog(@"%@", [carObj valueForKeyPath:@"warrantyPlans"]);
            [carObj valueForKeyPath:@"warrantyPlans"] = response;
        }];
    }

Answer 1

[carObj valueForKeyPath:@"warrantyPlans"] is an expression that has a value like literally 42 is an expression that has a value. It's not an expression like foo that is a variable that has a value of 42 and can be changed by using it on the left hand side of an equals sign.

To change it you want:

[carObj setValue:response forKeyPath:@"warrantyPlans"]

These two are called the generic getter valueForKey: and the generic setter setValue:forKey: in the KeyValueCoding guide: https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/KeyValueCoding/index.html