Rails applications can end up with models that get way too big, and so far, the Ruby community response has been Service Objects. But sometimes app/services can turn into another junk drawer that doesn't help you build and make concepts for your Domain Model.

ActiveRecord::AssociatedObject takes that head on. Associated Objects are a new domain concept, a context object, that's meant to help you tease out collaborator objects for your Active Record models.

They're essentially POROs that you associate with an Active Record model to get benefits both in simpler code as well as automatic app/models organization.

Let's look at an example. Say you have a Post model that encapsulates a blog post in a Content-Management-System:

class Post < ApplicationRecord
end

You've identified that several things need to happen when a post gets published. But where does that behavior live; in Post? That might get messy.

If we put it in a classic Service Object, we've got access to a def call method and that's it — what if we need other methods that operate on the state? And then having PublishPost or a similar ad-hoc name in app/services can pollute that folder over time.

What if we instead identified a Publisher collaborator object, a Ruby class that handles publishing? What if we required it to be placed within Post:: to automatically help connote the object as belonging to and collaborating with Post? Then we'd get app/models/post/publisher.rb which guides naming and gives more organization in your app automatically through that convention — and helps prevent a junk drawer from forming.

This is what Associated Objects are! We'd define it like this:

# app/models/post/publisher.rb
class Post::Publisher < ActiveRecord::AssociatedObject
end

And then you can declare it in Post:

# app/models/post.rb
class Post < ApplicationRecord
  has_object :publisher
end

There isn't anything super special happening yet. Here's essentially what's happening under the hood:

class Post::Publisher
  attr_reader :post
  def initialize(post) = @post = post
end

class Post < ApplicationRecord
  def publisher = (@associated_objects ||= {})[:publisher] ||= Post::Publisher.new(self)
end

Note: due to Ruby's Object Shapes, we use a single @associated_objects instance variable that's assigned to nil on Post.new. This prevents Active Record's from ballooning into many different shapes in Ruby's internals. We've fixed this so you don't need to care, but this is what's happening.

See how we're always expecting a link to the model, here post?

Because of that, you can rely on post from the associated object:

class Post::Publisher < ActiveRecord::AssociatedObject
  def publish
    # `transaction` is syntactic sugar for `post.transaction` here.
    transaction do
      post.update! published: true
      post.subscribers.post_published post

      # There's also a `record` alias available if you prefer the more general reading version:
      # record.update! published: true
      # record.subscribers.post_published record
    end
  end
end

To further help illustrate how your collaborator Associated Objects interact with your domain model, you can forward callbacks.

Say we wanted to have our publisher automatically publish posts after they're created. Or we need to refresh a publishing after a post has been touched. Or what if we don't want posts to be destroyed if they're published due to HAHA BUSINESS rules?

So has_object can state this and forward those callbacks onto the Associated Object:

class Post < ActiveRecord::Base
  # Passing `true` forwards the same name, e.g. `after_touch`.
  has_object :publisher, after_touch: true, after_create_commit: :publish,
    before_destroy: :prevent_errant_post_destroy

  # The above is the same as writing:
  after_create_commit { publisher.publish }
  after_touch { publisher.after_touch }
  before_destroy { publisher.prevent_errant_post_destroy }
end

class Post::Publisher < ActiveRecord::AssociatedObject
  def publish
  end

  def after_touch
    # Respond to the after_touch on the Post.
  end

  def prevent_errant_post_destroy
    # Passed callbacks can throw :abort too, and in this example prevent post.destroy.
    throw :abort if haha_business?
  end
end

Since has_object eager-loads the Associated Object class, you can also move any integrating code into a provided extension block:

class Post::Publisher < ActiveRecord::AssociatedObject
  extension do
    # Here we're within Post and can extend it:
    has_many :contracts, dependent: :destroy do
      def signed? = all?(&:signed?)
    end

    def self.with_contracts = includes(:contracts)

    after_create_commit :publish_later, if: -> { contracts.signed? }

    # An integrating method that operates on `publisher`.
    private def publish_later = publisher.publish_later
  end
end

This is meant as an alternative to having a wrapping ActiveSupport::Concern in yet-another file like this:

class Post < ApplicationRecord
  include Published
end

# app/models/post/published.rb
module Post::Published
  extend ActiveSupport::Concern

  included do
    has_many :contracts, dependent: :destroy do
      def signed? = all?(&:signed?)
    end

    has_object :publisher
    after_create_commit :publish_later, if: -> { contracts.signed? }
  end

  class_methods do
    def with_contracts = includes(:contracts)
  end

  # An integrating method that operates on `publisher`.
  private def publish_later = publisher.publish_later
end

The primary benefit for right now is that by focusing the concept of namespaced Collaborator Objects through Associated Objects, you will start seeing them when you're modelling new features and it'll change how you structure and write your apps.

This is what @natematykiewicz found when they started using the gem (we'll get to ActiveJob::Performs soon):

We're running ActiveRecord::AssociatedObject and ActiveJob::Performs (via the associated object) in 3 spots in production so far. It massively improved how I was architecting a new feature. I put a PR up for review and a coworker loved how organized and easy to follow the large PR was because of those 2 gems. I'm now working on another PR in our app where I'm using them again. I keep seeing use-cases for them now. I love it. Thank you for these gems!

Anyone reading this, if you haven't checked them out yet, I highly recommend it.

And about a month later it was still holding up:

Just checking in to say we've added like another 4 associated objects to production since my last message. ActiveRecord::AssociatedObject + ActiveJob::Performs is like a 1-2 punch super power. I'm a bit surprised that this isn't Rails core to be honest. I want to migrate so much of our code over to this. It feels much more organized and sane. Then my app/jobs folder won't have much in it because most jobs will actually be via some associated object's _later method. app/jobs will then basically be cron-type things (deactivate any expired subscriptions).

Here's what @nshki found when they tried it:

Spent some time playing with @kaspth's ActiveRecord::AssociatedObject and ActiveJob::Performs and wow! The conventions these gems put in place help simplify a codebase drastically. I particularly love ActiveJob::Performs—it helped me refactor out all ApplicationJob classes I had and keep important context in the right domain model.

Let's look at testing, then we'll get to passing these POROs to jobs like the quotes mentioned!

Follow the app/models/post.rb and app/models/post/publisher.rb naming structure in your tests and add test/models/post/publisher_test.rb.

Then test it like any other object:

# test/models/post/publisher_test.rb
class Post::PublisherTest < ActiveSupport::TestCase
  # You can use Fixtures/FactoryBot to get a `post` and then extract its `publisher`:
  setup { @publisher = posts(:one).publisher }
  setup { @publisher = FactoryBot.build(:post).publisher }

  test "publish updates the post" do
    @publisher.publish
    assert @publisher.post.reload.published?
  end
end

Associated Objects include GlobalID::Identification and have automatic Active Job serialization support that looks like this:

class Post::Publisher < ActiveRecord::AssociatedObject
  class PublishJob < ApplicationJob
    def perform(publisher) = publisher.publish
  end

  def publish_later
    PublishJob.perform_later self # We're passing this PORO to the job!
  end

  def publish
    # …
  end
end

This pattern of a job perform consisting of calling an instance method on a sole domain object is ripe for a convention, here's how to do that.

If you also bundle active_job-performs in your Gemfile like this:

gem "active_job-performs"
gem "active_record-associated_object"

Every Associated Object (and Active Records too) now has access to the performs macro, so you can do this:

class Post::Publisher < ActiveRecord::AssociatedObject
  performs queue_as: :important
  performs :publish
  performs :retract

  def publish
  end

  def retract(reason:)
  end
end

which spares you writing all this:

class Post::Publisher < ActiveRecord::AssociatedObject
  # `performs` without a method defines a general job to share between method jobs.
  class Job < ApplicationJob
    queue_as :important
  end

  # Individual method jobs inherit from the `Post::Publisher::Job` defined above.
  class PublishJob < Job
    # Here's the GlobalID integration again, i.e. we don't have to do `post.publisher`.
    def perform(publisher, *, **) = publisher.publish(*, **)
  end

  class RetractJob < Job
    def perform(publisher, *, **) = publisher.retract(*, **)
  end

  def publish_later(*, **) = PublishJob.perform_later(self, *, **)
  def retract_later(*, **) = RetractJob.perform_later(self, *, **)
end

Note: you can also pass more complex configuration like this:

performs :publish, queue_as: :important, discard_on: SomeError do
  retry_on TimeoutError, wait: :exponentially_longer
end

See the ActiveJob::Performs README for more details.

We've got automatic Kredis integration for Associated Objects, so you can use any kredis_* type just like in Active Record classes:

class Post::Publisher < ActiveRecord::AssociatedObject
  kredis_datetime :publish_at # Uses a namespaced "post:publishers:<post_id>:publish_at" key.
end

If you have a namespaced Active Record like this:

# app/models/post/comment.rb
class Post::Comment < ApplicationRecord
  belongs_to :post
  belongs_to :creator, class_name: "User"

  has_object :rating
end

You can define the associated object in the same way it was done for Post::Publisher above, within the Post::Comment namespace:

# app/models/post/comment/rating.rb
class Post::Comment::Rating < ActiveRecord::AssociatedObject
  def good?
    # A `comment` method is generated to access the associated comment. There's also a `record` alias available.
    comment.creator.subscriber_of? comment.post.creator
  end
end

And then test it in test/models/post/comment/rating_test.rb:

class Post::Comment::RatingTest < ActiveSupport::TestCase
  setup { @rating = posts(:one).comments.first.rating }
  setup { @rating = FactoryBot.build(:post_comment).rating }

  test "pretty, pretty, pretty, pretty good" do
    assert @rating.good?
  end
end

We support Active Record models with composite primary keys out of the box.

Just setup the associated objects like the above examples and you've got GlobalID/Active Job and Kredis support automatically.

This gem is relatively tiny and I'm not expecting more significant changes on it, for right now. It's unofficial and not affiliated with Rails core.

Though it's written and maintained by an ex-Rails core person, so I know my way in and out of Rails and how to safely extend it.

Install the gem and add to the application's Gemfile by executing:

$ bundle add active_record-associated_object

If bundler is not being used to manage dependencies, install the gem by executing:

$ gem install active_record-associated_object

After checking out the repo, run bin/setup to install dependencies. Then, run rake test to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and the created tag, and push the .gem file to rubygems.org.

Bug reports and pull requests are welcome on GitHub at https://github.com/kaspth/active_record-associated_object.

The gem is available as open source under the terms of the MIT License.