descendants_describable.gem

A DSL for quickly modelling large sets of STI models with shared behaviors.

This gem provides a shorthand DSL for including re-usable sets of mixin modules across a large set of STI models. If you find yourself with a folder of subclasses full of boilerplate and shared functionality, this gem will let you replace those files with one or a few small files that read more descriptively, like so:

MySuperClass.describe_descendants_with(MySuperClassMixins) do
  type :some_type
  type :another_subtype do
    email_addressable
  end

  type :some_domain_event do
    email_addressable
    payment_validatable
  end
end

Why?

My blog post describes the evolution of this gem at Hired pretty well.

TL;DR imagine the example in the section below with a marketplace’s domain described therein.

Usage

Here’s a piece of the real-world example that gave life to this gem:

  # config/initializers/activities.rb

  Activity.describe_descendants_with(Activity::Descriptors) do
    type :completed_survey do
      user_required
    end

    type :bid_on_developer do
      approved_employers_only
      target_required
    end

    type :auction_membership_confirmed do
      approved_developers_only
      actor_unique_to_auction
      target_required
    end
    # ... others omitted for brevity ...
end

Given this example, one is describing the descendants of an Activity class (that also live on the activities table with a type column for STI), the following is happening.

Each “type” declaration instantiates a new subclass by that name camelized.
Each method called inside the block provided to a “type” declaration will include into that new subclass a module by that method name camelized in the provided descriptor namespace.

So the above is basically shorthand for this:

# in models/completed_survey.rb
class CompletedSurvey < Activity
  include Activity::Descriptors::UserRequired
end

# in models/bid_on_developer.rb
class BidOnDeveloper < Activity
  include Activity::Descriptors::ApprovedEmployersOnly
  include Activity::Descriptors::TargetRequired
end

# in models/auction_membership_confirmed.rb
class AuctionMembershipConfirmed < Activity
  include Activity::Descriptors::ApprovedEmployersOnly
  include Activity::Descriptors::ActorUniqueToAuction
  include Activity::Descriptors::TargetRequired
end