inverse_of.rb

require "bundler/inline"

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

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

  gem 'activerecord', '6.1'
  gem "sqlite3"
end

require 'active_record'
require 'minitest/autorun'

# 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 :accounts do |t|
      t.timestamps
      t.string :name
    end

    create_table :users do |t|
      t.references :account, foreign_key: true, index: true
      t.timestamps
    end
end

class Account < ActiveRecord::Base
  has_many :users
end

class User < ActiveRecord::Base
  belongs_to :account
end

class BugTest < Minitest::Test
  def test_inverse_of
    a = Account.create!(name: 'a')
    User.create!(account: a)
    a = Account.first
    u = a.users.first

    assert a.name == u.account.name
    a.name = 'a1'
    assert a.name == u.account.name
    u.account.name = 'a2'
    assert a.name == u.account.name
  end
end