Fix belongs_to association validation for virtual attributes

[kyanagi]

belongs_to associations in ActiveType::Object did not properly validate presence when ActiveRecord::Base.belongs_to_required_by_default = true and ActiveRecord.belongs_to_required_validates_foreign_key = false (Rails default).

require "bundler/inline"

gemfile do
  source "https://rubygems.org"
  gem "activerecord"
  gem "sqlite3"
  gem "active_type"
end

require "active_record"
require "sqlite3"
require "active_type"

ActiveRecord::Base.belongs_to_required_by_default = true
ActiveRecord.belongs_to_required_validates_foreign_key = false

ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")

ActiveRecord::Schema.define do
  create_table :shops do |t|
  end

  create_table :items do |t|
    t.references :shop, null: false, foreign_key: true
  end
end

class Shop < ActiveRecord::Base
end

class Item < ActiveRecord::Base
  belongs_to :shop
end

class X < ActiveType::Object
  attribute :shop_id, :integer
  belongs_to :shop
end

p Item.new.valid?              #=> false
p Item.new(shop_id: -1).valid? #=> false
p X.new.valid?                 #=> false
p X.new(shop_id: -1).valid?    #=> true

This is because the validates_presence_of validation uses ActiveType::Object#attribute_changed? and it returns always false for virtual attributes.

p X.new(shop_id: 1).attribute_changed?(:shop_id) #=> false

https://github.com/rails/rails/blob/v8.0.1/activerecord/lib/active_record/associations/builder/belongs_to.rb#L135

This commit implements ActiveType::VirtualAttributes#attribute_changed? and fixes the problem.

The same applies to ActiveType::Record.