Add InstanceMethods#swapped_id_previously_was

[wata727]

This PR adds a new instance method called swapped_id_previously_was. This allows you to get the previous swapped_id after the update/destroy:

employee = Employee.create!(name: 'John')
employee.swapped_id # => 1
employee.swapped_id_previously_was # => nil

employee.update!(name: 'Sam')
employee.swapped_id # => 2
employee.swapped_id_previously_was # => 1

employee.reload
employee.swapped_id # => 2
employee.swapped_id_previously_was # => nil

employee.destroy!
employee.swapped_id # => 3
employee.swapped_id_previously_was # => 2

This is useful for retrieving before/after values delta from an updated instance. The naming is inspired by ActiveModel::Dirty.

It is intentional that swapped_id_was is undefined because the swapped_id is not basically assumed to be rewritten from the outside.

[motsat]

LGTM 👍

[Dooor]

LGTM 🎉