RailsのActiveRecordなどを単体で動かしてpry-byebugでデバッグする

Railsのソースコード読んでても黒魔術がいっぱいで、どうやって動いてるのかよく分からんです。

rails newで素のサンプルアプリケーションを作って、 pry-byebug などのデバッカを差し込んでもよいんですけど、railsのバグ報告に使われてるissueテンプレート使うと1ファイルで書けるし、rubyはdockerで実行しちゃえばクリーンな環境でできそう。

元ネタは以下です。

pry-byebugの設定を追加で差し込んでおきます。

test.rb

# frozen_string_literal: true

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  git_source(:github) { |repo| "https://github.com/#{repo}.git" }

  # Activate the gem you are reporting the issue against.
  gem "activerecord", "6.0.0.rc2"
  gem "sqlite3"
  gem 'pry-byebug'
end

require "active_record"
require "minitest/autorun"
require "logger"
require 'pry'

if defined?(PryByebug)
  Pry.commands.alias_command 'c', 'continue'
  Pry.commands.alias_command 's', 'step'
  Pry.commands.alias_command 'n', 'next'
  Pry.commands.alias_command 'f', 'finish'
end

binding.pry

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :posts, force: true do |t|
  end

  create_table :comments, force: true do |t|
    t.integer :post_id
  end
end

class Post < ActiveRecord::Base
  has_many :comments
end

class Comment < ActiveRecord::Base
  belongs_to :post
end

class BugTest < Minitest::Test
  def test_association_stuff
    post = Post.create!
    post.comments << Comment.create!

    assert_equal 1, post.comments.count
    assert_equal 1, Comment.count
    assert_equal post.id, Comment.first.post.id
  end
end

$ docker run -it --rm -v $(pwd):/tmp/work ruby:2.6.3 /bin/bash
root@27dd893231b5:/# ruby /tmp/work/test.rb

From: /tmp/work/test.rb @ line 31 :

    26: end
    27:
    28: binding.pry
    29:
    30: # This connection will do for database-independent bug reports.
 => 31: ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
    32: ActiveRecord::Base.logger = Logger.new(STDOUT)
    33:
    34: ActiveRecord::Schema.define do
    35:   create_table :posts, force: true do |t|
    36:   end

あとはご自由に。