Ответ 1
Это часть ActiveModel:: Dirty Вы можете увидеть его здесь https://github.com/rails/rails/blob/af64ac4e5ce8406137d5520fa88e8f652ab703e9/activemodel/lib/active_model/dirty.rb#L146 Пример
person = Person.find_by_name('Uncle Bob')
person.changed? # => false
Измените имя:
person.name = 'Bob'
person.changed? # => true
person.name_changed? # => true
#method _was return prev attribute value
person.name_was # => 'Uncle Bob'
person.name_change # => ['Uncle Bob', 'Bob']
person.name = 'Bill'
person.name_change # => ['Uncle Bob', 'Bill']