https://github.com/rails/rails/issues/29702

activerecord_bug_report_template.rb

begin
  require "bundler/inline"
rescue LoadError => e
  $stderr.puts "Bundler version 1.10 or later is required. Please update your Bundler"
  raise e
end

gemfile(true) do
  source "https://rubygems.org"
  gem "rails", "4.2.7.1"
  gem "arel", github: "rails/arel"
  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
  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!
    post.comments << Comment.create!
    post.comments << Comment.create!
    
    assert_equal 3, post.comments.count
    assert_equal [1, 2, 3], post.comments.pluck(:id)
    assert_equal [1, 2, 3], post.comments.map(&:id)
    
    search_comments = [Comment.last]
    search_result = Post.includes(:comments).find_by(comments: { id: search_comments })

    assert_equal 3, search_result.comments.count
    assert_equal [1, 2, 3], search_result.comments.pluck(:id)
    # Fails, should be same array as the line above.
    assert_equal [1, 2, 3], search_result.comments.map(&:id)
  end
end

Finished in 0.049941s, 20.0236 runs/s, 100.1181 assertions/s.

  1) Failure:
BugTest#test_association_stuff [bug.rb:55]:
Expected: [1, 2, 3]
  Actual: [3]

1 runs, 5 assertions, 1 failures, 0 errors, 0 skips