ActiveRecord::Enumで陥りがちなミスを避ける ref: http://qiita.com/ainame/items/a097558e74bf7d5087b9

file0.txt

class NotifyEndpoint
  enum platform: [:apns, :gcm]
end

file1.txt

# OK: newやcreateでは渡せる
endpoint = NotifyEndpoint.new(platform: :apns)

# OK: platform=メソッドにも渡せる
endpoint.platform = :gcm

# OK: apnsというスコープは定義されている
endpoints = NotifyEndpoint.apns

# NG: whereに文字列やシンボルを渡しても正しく動作しない
#     しかしエラーが出ずに0としてSELECTされる!!
endpoints = NotifyEndpoint.where(platform: :gcm)
#=> SELECT * FROM notify_endoints WHERE platform = 0;

# OK: NotifyEndpoint.platformsから、enumの序数を取得してwhereで指定する
endpoints = NotifyEndpoint.where(platform: NotifyEndpoint.platforms[:gcm])
#=> SELECT * FROM notify_endoints WHERE platform = 1;

file2.txt

if defined?(ActiveRecord) &&  defined?(::ActiveRecord::Enum)
  module ActiveRecord
    module Enum
      module Scoping
        def self.extended(base)
          ::ActiveRecord::Enum.alias_method_chain(:enum, :scoping)
        end
      end

      def enum_with_scoping(definitions)
        enum_without_scoping(definitions)
        define_scoping_method(definitions)
      end

      private
      def define_scoping_method(definitions)
        definitions.each do |name, values|
          scoping_method_name =  "#{name}_as".to_sym
          scope(scoping_method_name, - > (item) { enum_scope(name, item) }})
        end
      end
    end

    def enum_scope(enum_name, item_name)
      query_hash =  {}
      query_hash[enum_name.to_sym] =  send(enum_name.to_s.pluralize)[item_name]
      where(query_hash)
    end
  end

  ::ActiveRecord::Base.extend(::ActiveRecord::Enum::Scoping)
end

file3.txt

endpoints = NotifyEndpoint.platform_as(:gcm)
#=> SELECT * FROM notify_endpoints WHERE platform = 1;