rails3.2 と 4.1 での ActiveRecord select の違い

rails4.1 では ActiveRecord select で id を指定していないにもかかわらず、オブジェクトに id: nil を含んでしまうようです。
to_json する際に注意が必要そう。

例

desc users;
+------------+--------------+------+-----+---------+----------------+
| Field      | Type         | Null | Key | Default | Extra          |
+------------+--------------+------+-----+---------+----------------+
| id         | int(11)      | NO   | PRI | NULL    | auto_increment |
| nickname   | varchar(255) | YES  |     | NULL    |                |
| birthday   | datetime     | YES  |     | NULL    |                |
| created_at | datetime     | NO   |     | NULL    |                |
| updated_at | datetime     | NO   |     | NULL    |                |
+------------+--------------+------+-----+---------+----------------+

rails3.2

> User.select([:nickname, :birthday])
=> [#<User nickname: "hoge", birthday: "2014-08-18 06:08:35">, #<User nickname: "hoge2", birthday: "2014-08-18 06:08:39">, #<User nickname: "hoge3", birthday: "2014-08-18 06:08:42">]

> User.select([:nickname, :birthday]).to_json
=> "[{\"birthday\":\"2014-08-18T06:08:35Z\",\"nickname\":\"hoge\"},{\"birthday\":\"2014-08-18T06:08:39Z\",\"nickname\":\"hoge2\"},{\"birthday\":\"2014-08-18T06:08:42Z\",\"nickname\":\"hoge3\"}]"

rails4.1

> User.select([:nickname, :birthday])
=> #<ActiveRecord::Relation [#<User id: nil, nickname: "hoge", birthday: "2014-04-30 03:01:24">, #<User id: nil, nickname: "hoge2", birthday: "2014-03-31 03:01:31">, #<User id: nil, nickname: "hoge3", birthday: "2013-12-08 03:01:40">]>

> User.select([:nickname, :birthday]).to_json
=> "[{\"nickname\":\"hoge\",\"birthday\":\"2014-04-30T03:01:24.000Z\",\"id\":null},{\"nickname\":\"hoge2\",\"birthday\":\"2014-03-31T03:01:31.000Z\",\"id\":null},{\"nickname\":\"hoge3\",\"birthday\":\"2013-12-08T03:01:40.000Z\",\"id\":null}]"