Trying to reproduce https://github.com/rails/rails/issues/9548

gistfile1.rb

require 'bundler'
Bundler.setup(:default)

require 'pg'
require_relative 'lib/active_record'
require 'minitest/autorun'
require 'logger'

require 'byebug'

ActiveRecord::Base.establish_connection(adapter: 'postgresql', database: 'test123')
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :hosts, force: true do |t|
  end
  create_table :links, force: true do |t|
    t.integer :host_id
    t.integer :page_id
  end
  create_table :pages, force: true do |t|
  end
end

class Host < ActiveRecord::Base
  has_many :links
  has_many :pages, through: :links
end

class Link < ActiveRecord::Base
  belongs_to :host
  belongs_to :page
end

class Page < ActiveRecord::Base
  has_many :links
  has_many :hosts, through: :links
end

class BugTest < Minitest::Test
  def test_hmt
    host = Host.new
    page = Page.new
    host.pages.include? page
    
    refute host.pages.include?(page)

    host.pages << page

    assert host.pages.include?(page)

    host.pages.delete page

    refute host.pages.include?(page)
  end
end