named_scope_subset_helper.rb

def should_be_a_subset(superset, records_selected_by_scope, &condition)
  flunk "Your superset is empty" if superset.empty?
  flunk "Your scope did not select any records" if records_selected_by_scope.empty?

  records_selected_by_block, records_excluded_by_block = superset.partition(&condition)
  flunk "Your test condition did not select any records" if records_selected_by_block.empty?
  flunk "Your test condition did not exclude any records" if records_excluded_by_block.empty?

  records_selected_by_scope.map(&:id).should =~ records_selected_by_block.map(&:id)
end

Examples become even more readable if you make a custom matcher out of this:

RSpec::Matchers.define :be_a_subset_of do |superset, condition|
  match do |subset|
    should_be_a_subset(superset, subset, &condition)
  end
end

usage:

User.registered.should be_a_subset_of(User.all, lambda {|u| !u.registered_at.nil? })

i was inspired by this post to make an analogous matcher for ordering scopes

https://gist.github.com/seako/7632799