rails5でactive_hashのモデルをbelongs_toで参照できるようにする

動作確認バージョン

Gemfile.lock

rails (5.0.0.1)
active_hash (1.4.1)

親モデル

# == Schema Information
#
# Table name: parents
#
#  blood_type_id :integer
#
ApplicationRecord.extend ActiveHash::Associations::ActiveRecordExtensions

class Parent < ApplicationRecord
  belongs_to_active_hash :blood_type
end

active_hashモデル

class BloodType < ActiveHash::Base
  include ActiveHash::Associations
  has_one :parent

  fields :label, :name
  add id: 1, label: 'A型',   name: 'a'
  add id: 2, label: 'B型',   name: 'b'
  add id: 3, label: 'O型',   name: 'o'
  add id: 4, label: 'AB型',  name: 'ab'
end

使用例

irb(main):001:0> parent = Parent.new(blood_type_id: 4)
=> #<Parent id: nil, blood_type_id: 4,created_at: nil, updated_at: nil>

irb(main):002:0> parent.blood_type.label
=> "AB型"