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Georgy Yuriev
Georgy Yuriev

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Simple decorators with SimpleDelegator

#ruby #rails #programming

Decorator is a powerful pattern that helps us keep our models clean while adding presentation logic. Today, I want to share my approach to implementing decorators in Ruby on Rails.

I was inspired by a great article:
Build a minimal decorator with Ruby in 30 minutes
by Rémi - @[email protected]
Thanks to Ruby Weekly for sharing it!

What if we took a different approach? Let's solve this problem in reverse, like in the Tenet movie where time flows backwards 😈

Imagine we have

  • a Post model with a status enum column,
  • an Author model with first_name and last_name fields.

Let's create some decorators to enhance these models.

# app/decorators/post_decorator.rb

class PostDecorator < SimpleDelegator
  STATUS_COLORS = {
    published: :green,
    draft: :indigo
    archived: :gray,
    deleted: :red
  }.freeze

  def status_color = STATUS_COLORS[status.to_sym]
end
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# app/decorators/author_decorator.rb

class AuthorDecorator < SimpleDelegator
  def full_name
    name = [first_name, last_name].compact.join(' ')
    name.presence || 'Guest'
  end
end
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Now we can use it this way:

@posts = Post.all.map { PostDecorator.new(it) }
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<%= AuthorDecorator.new(post.author).full_name %>
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It adds so much boilerplate to the source code - I really dislike these wrappers. Let's clean this up adding method_missing method to all our models.

Image description

# app/models/application_record.rb

class ApplicationRecord < ActiveRecord::Base
  self.abstract_class = true

+  include Decoratable
end
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# app/models/concerns/decoratable.rb

module Decoratable
  extend ActiveSupport::Concern

  def respond_to_missing?(method_name, ...)
    return false unless decorator_class

    decorator_class.instance_methods.include?(method_name)
  end

  def method_missing(method_name, ...)
    return super unless respond_to_missing?(method_name)

    decorated_instance.public_send(method_name, ...)
  end

  private

  def decorator_class
    @decorator_class ||= "#{model_name}Decorator".safe_constantize
  end

  def decorated_instance
    @decorated_instance ||= decorator_class.new(self)
  end
end
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This will check if the method exists in the corresponding decorator before throwing an error, allowing us to use decorator methods as if they were defined in the models themselves!

- @posts = Post.all.map { PostDecorator.new(it) }
+ @posts = Post.all

- <%= AuthorDecorator.new(post.author).full_name %>
+ <%= post.author.full_name %>
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This approach gives us the best of both worlds: clean models and convenient access to decorator methods. The method_missing implementation acts as a bridge between our models and decorators, making the code more maintainable and easier to work with.

Much cleaner now, right?

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