gistfile1.rb

def should_be_a_subset(all_objects, constrained_scope, &condition)
  flunk "Your superset is empty" if all_objects.empty?
  flunk "Your constrained set is empty" if constrained_scope.empty?

  objects_for_which_condition_is_true, objects_for_which_condition_is_false = all_objects.partition(&condition)
  objects_for_which_condition_is_true.should_not be_empty
  objects_for_which_condition_is_false.should_not be_empty

  constrained_scope.map(&:id).sort.should == objects_for_which_condition_is_true.map(&:id).sort

  (all_objects.map(&:id) - constrained_scope.map(&:id)).should == objects_for_which_condition_is_false.map(&:id)
end