user.rb

# == Schema Information
#
# Table name: users
#
#  id         :integer         not null, primary key
#  active     :boolean
#  created_at :datetime
#  updated_at :datetime
#

class User < ActiveRecord::Base
  named_scope :active, :conditions => { :active => true }
end

user_spec.rb

require File.expand_path(File.dirname(__FILE__) + '/../spec_helper')

describe User do

  it "should get all active users" do
    active   = User.create!(:active => true)
    inactive = User.create!(:active => false)

    subject  = User.active
    subject.should be_all(&:active)

    inverse = User.all - subject

    inverse.none?(&:active).should be_true
    inverse.any?(&:active).should be_false

    # inverse.should be_none(&:active)
    inverse.should_not be_any(&:active)
  end

end