Using Metaprogramming to refactor code ref: http://qiita.com/vinhnguyenleasnet/items/3c96930a7b48786e81cf

file0.rb

    class User < ActiveRecord::Base
        validate_inclusion_of :activation_state, :in => ["pending", "active"]

        def self.pending_users
            find :all, :conditions => {:activation_state => 'pending'}
        end

        def self.active_users
            find :all, :conditions => {:activation_state => 'active'}
        end

        def pending?
            self.activation_state == 'pending'
        end

        def active?
            self.activation_state == 'active'
        end
    end

file1.rb

    class User < ActiveRecord::Base
        STATES = ['pending', 'active']
        validate_inclusion_of :activation_state, :in => STATES

        class <<self
            STATES.each do |state|
                define_method "#{state}_users" do
                    find :all, :conditions => {:activation_state => state}
                end
            end
        end

        STATES.each do |state|
            define_method "#{state}?" do
                self.activation_state == state
            end
        end
    end