Delegated relational composition for OR'ing polymorphic ActiveRecord associations

taskable.rb

module Taskable
  extend ActiveSupport::Concern
  included do
    has_many :tasks, as: :taskable
  end

  def tasks
    task_relations.inject(super) do |this, rel|
      this.or(rel.extending(AllTasks).all_tasks)
    end
  end

  def task_relations
    []
  end

  module AllTasks
    def all_tasks
      Task.where(taskable: self)
    end
  end
end

class Task < ApplicationRecord
  belongs_to :taskable, polymorphic: true
end

class Team < ApplicationRecord
  include Taskable
  has_many :accounts
  has_many :businesses

  def task_relations
    super | [accounts, businesses]
  end
end

class Account < ApplicationRecord
  include Taskable
  belongs_to :team
end

class Business < ApplicationRecord
  include Taskable
  belongs_to :team
end

Originally written in response to https://www.reddit.com/r/rails/comments/cchp51/is_this_sql_worth_writing_in_arel_ar_etc/. This was a fun little exercise and ultimately redundant because I then realised it is equivalent to writing

class Team < ApplicationRecord
  def all_tasks
    Task.where(taskable: [self, accounts, businesses])
  end

because Rails does the right thing with expanding the polymorphic subqueries. However, it does illustrate of couple of nice techniques, should you need them.