Rails #41986 Reproduction

rails_41986.rb

# frozen_string_literal: true

# In a many-to-many relationship, when setting values using
# ids, and nested_attributes, if the attributes come first,
# they are overriden by the ids. If the ids come first, the
# attributes are included too!
require "bundler/inline"

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

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

  # gem "rails", github: "rails/rails", branch: "main"
  gem "rails", "6.1.3.1"
  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

  create_table :authors, force: true do |t|
    t.string :name
  end

  create_join_table(:posts, :authors)
end

class Post < ActiveRecord::Base
  has_many :comments
  has_and_belongs_to_many :authors
  accepts_nested_attributes_for :comments
  accepts_nested_attributes_for :authors
end

class Author < ActiveRecord::Base
  has_and_belongs_to_many :posts
  accepts_nested_attributes_for :posts
end

class Comment < ActiveRecord::Base
  belongs_to :post
end

class BugTest < Minitest::Test
  def test_one_to_many_association
    comment = Comment.create! text: 'Hello world'
    post = Post.create!(comment_ids: [comment.id],
                        comments_attributes: [{ text: "Hello World is the 'first!1!' of the Programming World" }])
    post.reload

    assert_equal 1, Post.count
    assert_equal 2, post.comments.count
    assert_equal 2, Comment.count
    assert_equal post.id, Comment.second.post.id
    assert_equal post.id, Comment.first.post.id
    
  end

  def test_many_to_many_association_attributes_first
    author = Author.create! name: 'ChaelCodes'
    post = Post.create!(authors_attributes: [{ name: 'Twitch Chat' }], author_ids: [author.id])
    post.reload

    assert_equal 1, Post.count
    assert_equal 2, Author.count
    assert_equal 2, post.authors.count
    assert_includes Author.second.post_ids, post.id
    assert_includes Author.first.post_ids, post.id
  end

  def test_many_to_many_association_ids_first
    author = Author.create! name: 'ChaelCodes'
    post = Post.create!(author_ids: [author.id], authors_attributes: [{ name: 'Twitch Chat' }])
    post.reload

    assert_equal 1, Post.count
    assert_equal 2, Author.count
    assert_equal 2, post.authors.count
    assert_includes Author.second.post_ids, post.id
    assert_includes Author.first.post_ids, post.id
  end

  # It's a pun - Post Test, and post-test, get it?
  def post_test_teardown
    Post.destroy_all
    Author.destroy_all
    Comment.destroy_all
  end
  alias teardown post_test_teardown
end