I'm having a problem removing an item from a collection. I have a model called Class that represents a class of students, and that class can have many instances. I'm trying to remove classes that have no space left, but I keep getting the full set of instances.

$class->instances = $class->instances->reject(function($instance) {
    $registrations = $instance->registrations()->get();
    $instance->spaces_left = $instance->max_students - count($registrations);
    if($instance->spaces_left == 0) {
        return true;
     }
});

The weird thing is that if I assign the results of that reject function to something called $class-instances2 or any other name, it works just fine.