RailsのModelはPrimary keyに何も指定をしないとidカラムがPrimary keyとなります。(サロゲートキー)
idカラムの値はインクリメントされていくのですが、本記事はidカラムの値をインクリメントされる値から、ULIDに変更する方法です。

環境

Ruby: 2.7.2
Rails: 6.1.1
DB: PostgreSQL 13.1
ライブラリ: ULID

手順

カラムの型を変更(Migration)
Modelに追記
テスト

Migration

class CreateUsers < ActiveRecord::Migration[6.1]
  def change
    # idカラムをstringにする
    create_table :users, id: :string do |t|
      t.string :name, null: false

      t.timestamps
    end
  end
end

Model

models/user.rb


class User < ApplicationRecord
  include Identifiable
end

models/concerns/identifiable.rb

module Identifiable
  extend ActiveSupport::Concern

  included do
    # memo: primary_keyのデフォルトの値は`id`
    attribute primary_key, :string, default: -> { ULID.generate }
  end
end

実行結果

irb(main):001:0> User.new
=> #<User id: "01EX96S3Q185V0FSMFCCJBKN6P", name: nil, created_at: nil, updated_at: nil>

irb(main):006:0> User.create(name: 'name')
  TRANSACTION (0.2ms)  BEGIN
  User Create (0.5ms)  INSERT INTO "users" ("id", "name", "created_at", "updated_at") VALUES ($1, $2, $3, $4) RETURNING "id"  [["id", "01EX95SNJ85JX57H8F43JEAGPG"], ["name", "name"], ["created_at", "2021-01-30 08:15:51.880899"], ["updated_at", "2021-01-30 08:15:51.880899"]]
  TRANSACTION (3.6ms)  COMMIT
=> #<User id: "01EX95SNJ85JX57H8F43JEAGPG", name: "name", created_at: "2021-01-30 08:15:51.880899000 +0000", updated_at: "2021-01-30 08:15:51.880899000 +0000">

テスト

spec/models/user_spec.rb

require 'rails_helper'

RSpec.describe User, type: :model do
  it_behaves_like 'Identifiable'
end

spec/support/models/identifiable.rb


RSpec.shared_examples 'Identifiable' do
  describe '#primary_key' do
    context 'primary_keyカラムが`id`の場合' do
      subject { described_class.new.id }

      it { is_expected.to match(/[0-9A-Z]{26}/) }
    end
  end
end

参考

https://github.com/rails/rails/blob/5f3ff60084ab5d5921ca3499814e4697f8350ee7/activerecord/lib/active_record/attribute_methods/primary_key.rb#L89
https://api.rubyonrails.org/classes/ActiveRecord/Attributes/ClassMethods.html
https://github.com/shin1rok/ulid-rails-sample