An object that wraps a row in a database table or view, encapsulates the database access, and adds domain logic on that data.

An impedance-less match

• It looks and feels like Ruby, even when you need to go lower level to fine-tune things.
• It comes with fantastic and innovative answers for recurring needs when dealing with objects and relational persistence.

module Contact::Designatable
  extend ActiveSupport::Concern

  included do
    has_many :designations, class_name: "Box::Designation", dependent: :destroy
  end

  def designate_to(box)
    if box.imbox?
      # Skip designating to Imbox since it’s the default.
      undesignate_from(box.identity.boxes)
    else
      update_or_create_designation_to(box)
    end
  end

  def undesignate_from(box)
    designations.destroy_by box: box
  end

  def designation_within(boxes)
    designations.find_by box: boxes
  end

  def designated?(by:)
    designation_within(by.boxes).present?
  end

  private
    def update_or_create_designation_to(box)
      if designation = designation_within(box.identity.boxes)
        designation.update!(box: box)
      else
        designations.create!(box: box)
      end
    end
end

If your business logic is complex, you’ll soon want to use your object’s direct relationships, collections, inheritance, and so forth. These don’t map easily onto Active Record, and adding them piecemeal gets very messy.

class Topic
  include Entries

  #...
end

module Topic::Entries
  extend ActiveSupport::Concern

  included do
    has_many :entries, dependent: :destroy
    has_many :entry_attachments, through: :entries, source: :attachments
    has_many :receipts, through: :entries
    has_many :addressed_contacts, -> { distinct }, through: :entries
    has_many :entry_creators, -> { distinct }, through: :entries, source: :creator
    has_many :blocked_trackers, through: :entries, class_name: "Entry::BlockedTracker"
    has_many :clips, through: :entries
  end

  #...
end

class Box < ApplicationRecord
end

class Box::Imbox < Box
end

class Box::Trailbox < Box
end

class Box::Feedbox < Box
end

class Question < ApplicationRecord
  serialize :schedule, RecurrenceSchedule
end

class Contact < ApplicationRecord
  include Contactables
end

module Contact::Contactables
  extend ActiveSupport::Concern

  included do
    delegated_type :contactable, types: Contactable::TYPES, inverse_of: :contact, dependent: :destroy
  end
end

module Contactable
  extend ActiveSupport::Concern

  TYPES = %w[ User Extenzion Alias Person Service Tombstone ]

  included do
    has_one :contact, as: :contactable, inverse_of: :contactable, touch: true
    belongs_to :account, default: -> { contact&.account }
  end
end

class User < ApplicationRecord
  include Contactable
end

class Person < ApplicationRecord
  include Contactable
end

class Service < ApplicationRecord
  include Contactable
end

Encapsulation-friendly

class Reports::Users::ProgressController < ApplicationController
  def show
    @events = Timeline::Aggregator.new(Current.person, filter: current_page_by_creator_filter).events
  end
end

class Timeline::Aggregator
  def initialize(person, filter: nil)
    @person = person
    @filter = filter
  end

  def events
    Event.where(id: event_ids).preload(:recording).reverse_chronologically
  end

  private
    def event_ids
      event_ids_via_optimized_query(1.week.ago) || event_ids_via_optimized_query(3.months.ago) || event_ids_via_regular_query
    end

    # Fetching the most recent recordings optimizes the query enormously for large sets of recordings
    def event_ids_via_optimized_query(created_since)
      limit = extract_limit
      event_ids = filtered_ordered_recordings.where("recordings.created_at >= ?", created_since).pluck("relays.event_id")
      event_ids if event_ids.length >= limit
    end

    def event_ids_via_regular_query
      filtered_ordered_recordings.pluck("relays.event_id")
    end

    # ...
end

class Topic < ApplicationController
  include Accessible
end

module Topic::Accessible
  extend ActiveSupport::Concern

  included do
    has_many :accesses, dependent: :destroy
    scope :accessible_to, ->(contact) { not_deleted.joins(:accesses).where accesses: { contact: contact } }
  end

  # ...
end

class CollectionsController < ApplicationController
  def show
    @topics = @collection.topics.active.accessible_to(Acting.contact)
    # ...
  end
end

module Posting::Involving
  extend ActiveSupport::Concern

  DEFAULT_INVOLVEMENTS_JOIN = "INNER JOIN `involvements` USE INDEX(index_involvements_on_contact_id_and_topic_id) ON `involvements`.`topic_id` = `postings`.`postable_id`"
  OPTIMIZED_FOR_USER_FILTERING_INVOLVEMENTS_JOIN = "STRAIGHT_JOIN `involvements` USE INDEX(index_involvements_on_account_id_and_topic_id_and_contact_id) ON `involvements`.`topic_id` = `postings`.`postable_id`"

  included do
    scope :involving, ->(contacts, involvements_join: DEFAULT_INVOLVEMENTS_JOIN) do
      where(postable_type: "Topic")
        .joins(involvements_join)
        .where(involvements: { contact_id: Array(contacts).map(&:id) })
        .distinct
    end

    scope :involving_optimized_for_user_filtering, ->(contacts) do
      # STRAIGHT_JOIN ensures that MySQL reads topics before involvements
      involving(contacts, involvements_join: OPTIMIZED_FOR_USER_FILTERING_INVOLVEMENTS_JOIN)
        .use_index(:index_postings_on_user_id_and_postable_and_active_at)
        .joins(:user)
        .where("`users`.`account_id` = `involvements`.`account_id`")
        .select(:id, :active_at)
    end
  end
end

A restatement of the persistence and domain logic separation problem

An element is cohesive to the degree that its subelements are coupled. A perfectly cohesive elements requires all subelements to change at the same time.

Conclusions