Models

Models are objects that represent core entities of the app’s business logic. These are usually persisted and can be fetched and created as needed. They have unique keys for identification (usually a numeric value), and, most importantly perhaps, they are immutable. This is the key difference between this new Model layer of objects and the Active Record instances regularly referred to as models in typical Rails default apps.

Another difference between Models and Records is that, once instantiated, Models simply hold its attributes immutably, and they don’t have any capabilities to create or update any information in the persistence layer.

The collaboration between Repositories and Models is what allows Active Record to be completely hidden away from any other areas of the app. There are no references to Records in controllers, views, and anywhere else. Repositories are invoked instead, which in turn return read-only Models.

Let’s refactor the previous example of the Blog app to encapsulate the Article Record behind a Model and use those as return values in the Article Repository.

# app/models/article.rb

class Article
  attr_reader :id, :title, :body, :created_at, :updated_at

  def initialize(id:, title:, body:, created_at:, updated_at:)
    @id = id
    @title = title
    @body = body
    @created_at = created_at
    @updated_at = updated_at
  end
end

class ArticleRepository
  def all
    ArticleRecord.all.map { |record| to_model(record.attributes) }
  end

  def create(input)
    record = ArticleRecord.create!(title: input.title, body: input.body)
    to_model(record.attributes)
  end

  def find(id)
    record = ArticleRecord.find(id)
    to_model(record.attributes)
  end

  def update(id, input)
    record = ArticleRecord.find(id)
    record.update!(title: input.title, body: input.body)
    to_model(record.attributes)
  end

  def delete(id)
    record = ArticleRecord.find(id)
    record.destroy!
  end

  private

  def to_model(attributes)
    Article.new(**attributes.symbolize_keys)
  end
end

Note that since we were already using the Article Record as read-only objects, we were able to simply change the value returned by the Repository to a Model without breaking our controller and views.