Demonstration of the problem with counter_cache in Rails 5 (and 4)

post_test.rb

require 'test_helper'

# ActiveRecord::Schema.define(version: 20170310085807) do
#   create_table "forums", force: :cascade do |t|
#     t.string   "title"
#     t.integer  "posts_count"
#     t.datetime "created_at",  null: false
#     t.datetime "updated_at",  null: false
#   end
#
#   create_table "posts", force: :cascade do |t|
#     t.string   "subject"
#     t.integer  "forum_id"
#     t.text     "body"
#     t.datetime "created_at", null: false
#     t.datetime "updated_at", null: false
#   end
# end
#
#
# class Forum < ApplicationRecord
#   has_many :posts
# end
#
# class Post < ApplicationRecord
#   belongs_to :forum, counter_cache: true
# end

class PostTest < ActiveSupport::TestCase
  setup do
    @first = Forum.create(title: "The first forum.")
    @second = Forum.create(title: "The other forum.")

    @post = Post.create(subject: "The message.", body: "Nothing important.")
    @post.forum = @first
    @post.save
  end

  test "Repeatedly assigning the association increases the counter_cache count." do
    5.times do
      @post.forum = @second
      @post.reload
    end

    # We never saved the post, the first forum should have 1, and the second forum should have none.
    assert_equal [1, 0], [@first.reload.posts_count, @second.reload.posts_count]
    # Expected: [1, 0]
    #   Actual: [-4, 5]
  end

  test "The counter_cache gets updated when an association is assigned, not saved" do
    # The first forum correctly has 1 post.
    assert_equal 1, @first.reload.posts_count

    # Perhaps we should move the post?
    @post.forum = @second
    # Nah.  Note, we have not saved the post.
    @post.reload

    # The reload reset the forum id in the post.
    assert_equal @first.id, @post.forum_id

    # So the posts_count should not have increased, because we never saved the changed post.
    assert_equal 1, @first.reload.posts_count
    # Expected: 1
    #   Actual: 0
  end
end