I’ve bumped into this behavior many times, and always find it surprising. I don’t know if it’s a bug, or intended, but I would expect this to behave differently.
Given a blog, which has many posts & many comments through posts; when creating a new blog, assigning a preexisting post to its collection works fine, but the comments are “empty” from the blog’s perspective. This causes confusing behavior when attempting to add validations e.g. on whether a blog could be created with a post given some conditions of its comments, as it’s not possible to access the comments directly. It works on
# https://github.com/rails/rails/pull/29619#issuecomment-392583498 # Better approach than http://codesnik.github.io/rails/2015/09/03/activerecord-and-exists-subqueries.html require 'bundler/inline' gemfile(true) do source 'https://rubygems.org' gem 'rails', '6.0.3' gem 'pg' gem 'pry' end require 'active_record' require 'minitest/autorun' require 'logger' ActiveRecord::Base.establish_connection(adapter: 'postgresql', database: 'test_exists') ActiveRecord::Base.logger = Logger.new(STDOUT) ActiveRecord::Schema.define do create_table :blogs, force: true do |t| end create_table :posts, force: true do |t| t.references :blog t.string :body end create_table :comments, force: true do |t| t.references :post t.string :text end end class Blog < ActiveRecord::Base has_many :posts has_many :comments, through: :posts end class Post < ActiveRecord::Base has_many :comments end class Comment < ActiveRecord::Base belongs_to :post end class PostCommentsExistsInnerTest < ActiveSupport::TestCase def setup @post = Post.create! @comment = @post.comments.create!(text: 'Foo') @blog = Blog.new @blog.posts << @post end def test_post_is_included assert_includes @blog.posts, @post end def test_comment_is_included_through_posts assert_includes @blog.posts.comments, @comment end def test_comment_is_included assert_includes @blog.comments, @comment # fails on create, works if @blog is persisted. end end