s_ar_preload_scope.rb

require "bundler/inline"

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

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

  gem 'rails', '6.1'
  gem 'sqlite3'
end

require 'active_record'
require 'minitest/autorun'

ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :users do |t|
    t.references :user_comments
  end

  create_table :user_comments do |t|
    t.references :user
    t.references :comment
  end

  create_table :comments do |t|
    t.references :user_comments
    t.boolean :published, default: false
  end
end

class User < ActiveRecord::Base
  has_many :user_comments
  has_many :comments, through: :user_comments
  has_many :published_comments, -> { published }, through: :user_comments, source: :comment
end

class UserComment < ActiveRecord::Base
  belongs_to :comment
  belongs_to :user
end

class Comment < ActiveRecord::Base
  has_many :user_comments
  scope :published, ->{ where(published: true) }
end

class BugTest < Minitest::Test
  def test_preload_scoped
    u = User.new
    u.comments << Comment.new
    u.comments << Comment.new(published: true)
    u.save!

    # this pass
    assert_equal 1, User.preload(:published_comments, :comments).find(u.id).published_comments.size
    assert_equal 1, User.eager_load(:comments, :published_comments).find(u.id).published_comments.size
    # this fails, same applies for .includes which in this situation uses .preload as well
    assert_equal 1, User.preload(:comments, :published_comments).find(u.id).published_comments.size
  end
end