Trying to retrieve an Entity using an attribute of the Entity's relationship object as a search criteria in Core Data

Question

I am using Core Data in my application where I am trying to retrieve entities whose relationship objects attribute match my criteria. Unfortunately I am stuck here, because I am passing the id for the relationship object, but I am getting an error saying that the criteria I am passing is not being recognized as an attribute of the entity I am querying.

This is true, because my criteria is actually an attribute for the relationship object, and not an attribute of the entity that I am querying itself. How do I achieve this?

Here is my code:

    // Create fetch request
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:myEntityName inManagedObjectContext:context];
    [fetchRequest setEntity:entity];

    // Create predicate
    NSPredicate *pred = [NSPredicate predicateWithFormat:@"relationshipObjectId == %@", relationshipObjectId];//This is where I am having trouble
    [fetchRequest setPredicate:pred];

    NSArray *items = [context executeFetchRequest:fetchRequest error:&error];
    if ([items count]>0) {
        return items[0];
    } else {
        return nil;
    }

Can anyone see what it is I am doing wrong?

Answer 1

To find all related objects whose attribute matches a certain value, you would use a predicate like

[NSPredicate predicateWithFormat:@"rel.attr == %@", value]

where "rel" is the name of the relationship, and "attr" is the name of the attribute that should match the value.