replace_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", "6.1.7.3"
  gem "sqlite3"
end

require "active_record"
require "minitest/autorun"
require "logger"

# 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
    t.string :text
  end
end

class Post < ActiveRecord::Base
  has_many :comments
end

class Comment < ActiveRecord::Base
  belongs_to :post
end

class BugTest < Minitest::Test
  def test_replace1
    post = Post.create!
    ["no.1", "no.2", "no.3"].each do |s|
      post.comments << Comment.create!(text: s)
    end

    post = Post.first

    updated = post.comments[2]
    updated.text = "no.2"

    # try to no.1 -> replaced by new record
    # try to no.2 -> remove
    # try to no.3 -> no.2
    post.comments.replace([updated, Comment.new(text: "no.1")])

    #p Post.first.comments

    assert_equal [3, 4], Post.first.comments.order(:id).pluck(:id)
    # no changes for existing record
    assert_equal "no.3", Post.first.comments.find(updated.id).text
    assert_equal "no.1", Post.first.comments.order(:id).last.text
  end
end