Using key paths in NSPredicates
key-value-coding, key-value-observing, objective-c
Solution
The solution to your problem is to use the operator `@count` as in `@"parents.@count == 0"`.
Reading the exception we see that evaluating the predicate sent the message `-count` to a GTPerson object. Why?
Sending `-valueForKey:` to a collection (in your case the collection is the NSSet which was the result of evaluating the `parents` component of the key path) sends `-valueForKey:` to each object in the collection.
In this case that results in `-valueForKey: @"count"` being sent to each GTPerson instance, and GTPerson isn't key value coding compliant for count.
Instead, use the `@count` operator to evaluate the count of the collection when you want the count of the collection, rather than the value of the key `count` on all objects in the collection.
Problem
I have an NSDictionary which contains (my custom) GTPerson objects. GTPerson has an `NSMutableSet *parents` attribute, on which I use `@property` and `@synthesize`. Out of my NSDictionary, I want to filter all the GTPerson objects which don't have any parents, i.e. where the count of parents is 0. I'm using the following code: ``` NSPredicate *predicate = [NSPredicate predicateWithFormat:@"parents.count = 0"]; NSArray *np = [[people allValues] filteredArrayUsingPredicate:predicate]; ``` When I execute this, I receive the following error: `[<GTPerson 0x18e300> valueForUndefinedKey:]: this class is not key value coding-compliant for the key count.` `Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<GTPerson 0x18e300> valueForUndefinedKey:]: this class is not key value coding-compliant for the key count.'` Why is it trying to call `count` on GTPerson and not on its `parents` attribute?