class User < ActiveRecord::Base
  include ValidationContexts
 
  validation_context :default, :signup do
    validates_uniqueness_of :email, :allow_blank => true
    validates_presence_of   :email
  end
 
  validation_context :signup do
    validates_confirmation_of :password
    validates_presence_of     :password_confirmation
    validates_acceptance_of   :terms, :allow_nil => false
  end
 
  validation_context :default, :if => :password_changed? do
    validates_confirmation_of :password
    validates_presence_of :password_confirmation
  end
end

module ValidationContexts
  module ClassMethods
    # validation_context :signup {...}
    # validation_context :default, :signup {...}
    # validation_context :default, :if => :password_changed? {...}
    def validation_context(*args, &block)
      options = args.extract_options!
      guard = proc {|obj|
        if options[:if] && obj.respond_to?(options[:if])
          obj.send(options[:if])
        elsif options[:if].respond_to?(:call)
          options[:if].call(obj)
        else
          true
        end && args.include?(obj.validation_context)
      }
      with_options(:if => guard) do |o|
        o.instance_eval(&block)
      end
    end
  end
 
  module InstanceMethods
    def validation_context=(context)
      @validation_context = context
    end
    def validation_context
      @validation_context || :default
    end
  end
  
  def self.included(receiver)
    receiver.extend         ClassMethods
    receiver.send :include, InstanceMethods
  end
end