ddd_example.rb

# Inverting Dependencies: Hexagonal Architecture

# A Blog post
class Post
  def self.write_new_from(title, content)
    new(title, content)
  end

  private

  def initialize(title, content)
    raise 'Title cannot be empty' if title.empty?
    raise 'Content cannot be empty' if content.empty?

    @title = title
    @content = content
  end
end

# A Blog post repository
module PostRepository
  def by_id(_id)
    raise NotImplementedError
  end

  def add(_post)
    raise NotImplementedError
  end
end

# A PDO post repository
class PDOPostRepository
  include PostRepository

  def initialize(db)
    @db = db
  end

  def by_id(id)
    @db[:posts][id: id]
  end

  def add(post)
    @db.transaction do
      @db[:posts].insert title: post.title, content: post.content
    end
  end
end

# Application layer
class PostService
  def initialize(post_repository)
    @post_repository = post_repository
  end

  def create_post(title, content) # =========================> This can be subdivided by adding use case classes per action (CreatePost, DeletePost etc.)
    post = Post.write_new_from(title, content)
    @post_repository.add post
    post
  end
end

# Posts controller
class PostsController
  # :reek:TooManyStatements
  def update(request) # rubocop:disable Metrics/MethodLength
    db = PDO.new('mysql:host=localhost;dbname=my_database',
                 'a_username',
                 '4_p4ssw0rd',
                 init_command: 'SET NAMES utf8mb4')

    # post_repository = PDOPostRepository.new(db)
    # post_repository = PDOPostRepository.new(settings.db)
    post_repository = PDOPostRepository.new(Registry.post_repository_db)
    post_service = PostService.new(post_repository)

    begin
      post_service.create_post request['title'], request['content']

      add_flash 'notice', 'Post has been created successfully!'
    rescue
      add_flash 'error', 'Unable to create the post!'
    end
    render 'posts/posts.rhtml'
  end
end

# PostsController.new.update({'title' => 'foo', 'content' => 'bar'})

# Registry
module Registry
  def self.post_repository_db
    @post_repository_db ||= PDO.new(
      'mysql:host=localhost;dbname=my_database',
      'a_username',
      '4_p4ssw0rd',
      init_command: 'SET NAMES utf8mb4'
    )
  end
end