has_one, has_many, belongs_toでconditionsを動的にする

Rails has_many with dynamic conditionsより、ActiveRecord 3.1+ではprocを使う必要があるみたい。3.2.8で確認済み。

class User
  has_many :pictures
  has_one :profile_picture, class_name: "Picture", foreign_key: "user_id",
    conditions: proc { ["id = ?", self.profile_picture_id] }
end