BUG: counter_cache updates counter twice on assignment

failing_spec.rb

unless File.exist?('Gemfile')
  File.write('Gemfile', <<-GEMFILE)
    source 'https://rubygems.org'
    gem 'rails', github: 'rails/rails'
    gem 'arel', github: 'rails/arel'
    gem 'sqlite3'
  GEMFILE

  system 'bundle'
end

require 'bundler'
Bundler.setup(:default)

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 do |t|
    t.integer :comments_count
  end

  create_table :comments do |t|
    t.integer :post_id
  end
end

class Post < ActiveRecord::Base
  has_many :comments
end

class Comment < ActiveRecord::Base
  belongs_to :post, counter_cache: true
end

class BugTest < Minitest::Test
  def test_update_from_nil
    post = Post.create!
    comment = Comment.create!

    comment.post = post # this line causes the first increment
    comment.save!

    # NOTE: this doesn't work either
    #comment.update_attributes(post: post)

    # POW, comments_count is 2
    assert_equal 1, post.reload.comments_count
  end

  # note that the bug doesn't occur when updating a comment that was
  # previously assigned to another post
  def test_update_from_other_post
    post = Post.create!
    post2 = Post.create!
    comment = Comment.create!(post: post)
    assert_equal 1, post.reload.comments_count
    assert_equal nil, post2.reload.comments_count

    comment.post = post2
    comment.save!

    # POW, comments_count is 2
    assert_equal 0, post.reload.comments_count
    assert_equal 1, post2.reload.comments_count
  end
end