Question

I have a “Song” Entity and a “Tag” entity and they have a many to many relationship between them. A Song can have multiple Tags and a Tag can be applied to multiple Songs.

I want to check if a Song has a particular Tag associated with it. If the Song has the Tag associted with it, I want to show a checkmark in the table view.

For a similar logic, in Apple “TaggedLocations” sample code, the following check is made to check for the presence of the relationship.

if ([event.tags containsObject:tag]) {
    cell.accessoryType = UITableViewCellAccessoryCheckmark;
}   

This may be inefficient if there are a lot of Tags in the database as this will fetch all of them in the memory. Please correct me if I am wrong here.

Is there a more efficient way to check if the Song is associated with a particular Tag instead of checking in Song.Tags?

Answer 1

It’s actually pretty easy to do, if completely undocumented. You want to create a fetch request with a predicate that has a set operation. If we imagine that your Tag model has a property called tagValue, the predicate you care about is “ANY tags.tagValue == ‘footag'”

NSString *tagSearch = @"footag";

// However you get your NSManagedObjectContext.  If you use template code, it's from
// the UIApplicationDelegate
NSManagedObjectContext *context = [delegate managedObjectContext];

// Is there no shortcut for this?  Maybe not, seems to be per context...
NSEntityDescription *songEntity = [NSEntityDescription entityForName:@"Song" inManagedObjectContext:context];

NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:songEntity];

// The request looks for this a group with the supplied name
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"ANY tags.tagValue == %@", tagSearch];
[request setPredicate:predicate];

NSError *error = nil;
NSArray *results = [context executeFetchRequest:request error:&error];

[request release];