Question

Im trying to do a search for a child entity based on a field “guid”. The guid field is defined in the parent entity. I would like to find the child instance whos guid is equal to some value i pass in but it always returns a result of 0 values. Im assuming core data isn’t seeing the guid property since its defined in the parent entity.

Here is my predicate:

NSFetchRequest *request = [[NSFetchRequest alloc] init];

[request setEntity:[NSEntityDescription entityForName:CHILD_ENTITY inManagedObjectContext:managedObjectContext]];
[request setPredicate:[NSPredicate predicateWithFormat:@"(guid like '%@')", guid]];

NSError *error = nil;

NSArray *results = [managedObjectContext executeFetchRequest:request error:&error];

if (results != nil && results.count > 0)
    return YES;
else 
    return NO;

Is my predicate format wrong?

Answer 1

The solution is to use the full key path of the property.

[NSPredicate predicateWithFormat:@"parent.guid like %@", guid]

That is, if parent is the name of the property which points to the logical parent you speak of.

Of course, if by parent you mean literal inheritance (i.e. [child isKindOfClass:[parent class]] is YES), than the issue may have simply been the quotes. I have assumed that you meant a logical parent which is a different instance, and has a relationship to the child (I guess it’s OOP terms VS ORM terms, or something like that).

Also, no need for parenthesis or quoted %@ in predicates this simple.