Add dirty tracking of paranoia attributes

[pfeiffer]

This PR adds ActiveModel dirty tracking to the paranoid attributes.

See: #373

This is extremely useful if you have after_commit callbacks that should only run if certain attributes have changed, eg. if deleted_at is changed:

after_commit do
  keys_requiring_sync = %w[deleted_at email]
  if (previous_changes.keys & keys_requiring_sync).any?
    SyncUserWithExternalService.new(id).run!
  end
end

PR also supports custom attributes (eg. active and destroyed_at). In that case dirty tracking is enabled for all attributes.

[pfeiffer]

Will fix and squash commits to support Rails 5.0.0.

[BenMorganIO]

This is awesome. Thanks @pfeiffer !

[kmcgaire]

@pfeiffer whats the status on this?

[pfeiffer]

Sorry for not updating on this before now.

I didn't manage to get the dirty tracking working on Rails 5 since there has been some changes to the dirty tracking internals. The project I need this for isn't on Rails 5 yet, so it's not relevant for me.

Feel free to dig in and update make a new PR with Rails 5 support! :)

[dgmora]

@BenMorganIO It seems update_columns resets the previous changes in rails 5.1. If you add #{paranoia_column}_will_change after the update_columns, the tests pass. They don't pass with rails 5.0.x though, I couldn't find why 😢. Would you accept a PR that makes this pass for rails < 5.0 and >= 5.1 ?
Looking through rails code is a mess 😅

[BenMorganIO]

@dgmora I'd love to be able to look at it and maybe we can figure things out from there.

[dgmora]

@BenMorganIO Did you see #416 ? 🙂

[mathieujobin]

👀 is this still relevant to someone ?