Add an option to track changes to dynamic/virtual attributes (ActiveRecord 5.1+ behaviour)

[cblunt]

With the changes introduced in ActiveRecord 5.1, PaperTrail isn't able to track the changes of dynamic (virtual) attributes. After some investigation, I eventually ran into this issue on Rails - tracking the previous value of dynamic attributes is no longer supported.

This is breaking behaviour for an app I am upgrading from Rails 4 to Rails 5.2. In this particular case, the app is using ActsAsTaggableOn - changes to the tag_list attribute are no longer tracked. Instead, the previous value is always nil.

This will apply to any dynamic attribute that we try to track through PaperTrail.

Article.create(title: "Hello World", tag_list: "hello")

article = Article.first
article.update(tag_list: "world")

article.versions.last.changeset
=> {"tag_list" => [nil, ["world"]]   # Prior to AR5.1, {"tag_list" => [["hello"], ["world"]]

Describe the solution you'd like to build

My idea to workaround this is to cache the value of tag_list in a real column (cached_tag_list) and then restore its value into tag_list if and when the version is reified. This would require the addition of a after_reify callback, and ideally, a new dynamic[_attributes] option on the has_paper_trail method to easily declare which attributes should be cached.

The default would be to cache values into a column named cached_{attribute_name} on the model.

Example Usage

class Article < ApplicationRecord
  acts_as_taggable_on # for demonstration purposes. AATO adds a dynamic `tag_list` attribute
  
  has_paper_trail dynamic: [:tag_list]
end

Article.create(title: "Hello World", tag_list: "hello")
article = Article.first
article.update(tag_list: "world")

article.versions.last.changeset
=> { cached_tag_list: [["hello"], ["world"]], tag_list: [nil, ["world"]] }

article.versions.last.reify
=> #<Article id: 1, title: "My Article", tag_list: ["hello"], cached_tag_list: "hello">

Describe alternatives you've considered

I considered using the existing meta data feature to cache the value of the dynamic attributes, but this seems to be populated during the after_save callback, meaning that the previous value of the attribute is not available (attribute_before_last_save(:tag_list) is nil)

Edit: I'd also considered capturing the value of the attributes during the before_save callback and storing them to be written to the PaperTrail::Version record. If PaperTrail could do this, it would be a more elegant solution than adding additional fields to the database, but I haven't dug into PaperTrail enough to know if it's viable.

Resources

• Gist showing failing behaviour + Proposed workaround

[jaredbeck]

Hi Chris,

.. I am upgrading from Rails 4 to Rails 5.2 ..

So, I expected the provided script to pass against rails 4, but it fails with

-{"tag_list"=>[["hello"], ["world"]]}
+{"title"=>[nil, "My Article"], "tag_list"=>["", ["hello"]], "id"=>[nil, 1]}

I used AR 4.2 and AATO 5.

gemfile(true) do
  ruby '2.5.1'
  source 'https://rubygems.org'
  gem 'activerecord', '~> 4.2'
  gem 'minitest', '5.11.3'
  gem 'paper_trail', '10.0.1', require: false
  gem 'sqlite3', '1.3.13'
  gem 'acts-as-taggable-on', '5.0.0'
end

What am I missing? Thanks.

[jaredbeck]

Closing due to inactivity. To reopen, please provide the requested information, thanks!

[cblunt]

Hey @jaredbeck

The upgrade is coming from Rails 4.2 and old versions of PaperTrail (4.0.0) and ActsAsTaggableOn (3.5.0). There seems to be a change in AATO 4 which shows the same behaviour.

I'll do a bit more digging into this to see what changed in AATO as well. I spent a while trying to decide if this was a PT or AATO issue, but another test using my own dynamic attribute (without AATO) did the same thing. I'll try recreating that as well to see if I can isolate the issue.