Question

I have Week model that has Day as child objects. There is “days” relation property in Week model to access all associated Day objects. Day model has duration property.

How can I figure out sum of day’s durations for specified Week object? It would be great to have code example how to create a predicate object with @sum function.

Also is it possible to have “calculated” weekDuration property on Week class that given value
of sum of related day’s durations during fetch? It would be most elegant solution for that problems, but I don’t believe that is possible with CoreData.

Answer 1

Here is an example of how you would setup your query to find only weeks where the sum of the duration of the days is greater than 100.

NSManagedObjectContext *context = ...;
NSManagedObjectModel *model = ...;
NSFetchRequest *fr = [[NSFetchRequest alloc] init];
fr.entity = [model.entitiesByName objectForKey:@"Week"];

//This predicate will be compiled into pure SQL
fr.predicate = [NSPredicate predicateWithFormat:@"[e-mail] > 100"];

NSError *error = nil;
NSArray *results = [context executeFetchRequest:fr error:&error];
if (error) {
  NSLog(@"ERROR: %@", error);
}
NSLog(@"Results: %@", results);

You can actually implement the computed property in a similiar way, just add this to the NSManagedObject subclass backing your Week entity:

- (NSNumber *) duration {
  return [self valueForKeyPath:@"[e-mail]"];
}