Agent skill

Draper Decorators

This skill should be used when the user asks to "create a decorator", "write a decorator", "move logic into decorator", "clean logic out of the view", "isn't it decorator logic", "test a decorator", or mentions Draper, keeping views clean, or representation logic in decorators. Should also be used when editing *_decorator.rb files, working in app/decorators/ directory, questioning where formatting methods belong (models vs decorators vs views), or discussing methods like full_name, formatted_*, display_* that don't belong in models. Provides guidance on Draper gem best practices for Rails applications.

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SKILL.md

Draper Decorators for Rails

This skill provides guidance for creating effective Draper decorators in Rails applications.

Philosophy

Decorators implement separation of concerns between business logic (models) and presentation logic (views). A decorator wraps a model to add view-specific methods without polluting the model.

What belongs in decorators:

  • Date/time formatting (created_at.strftime("%B %d, %Y"))
  • String concatenation ("#{first_name} #{last_name}")
  • HTML generation (h.content_tag(:span, status, class: css_class))
  • Conditional rendering based on state
  • Number formatting (currency, percentages)
  • CSS class generation based on object state

What does NOT belong in decorators:

  • Business logic (validations, calculations, state changes)
  • Database queries (use includes in controllers)
  • Anything not directly related to presentation

Basic Structure

ruby
# app/decorators/user_decorator.rb
class UserDecorator < ApplicationDecorator
  delegate_all

  def full_name
    "#{first_name} #{last_name}"
  end

  def formatted_created_at
    created_at.strftime("%B %d, %Y")
  end

  def status_badge
    css_class = active? ? "badge-success" : "badge-secondary"
    h.content_tag(:span, status, class: "badge #{css_class}")
  end
end

Delegation Strategies

Option 1: delegate_all (Convenient)

Delegates all methods to the wrapped object via method_missing. Use for most decorators.

ruby
class ProductDecorator < ApplicationDecorator
  delegate_all

  def formatted_price
    h.number_to_currency(price)
  end
end

Option 2: Explicit Delegation (Strict)

Explicitly declare which methods to delegate. Use for larger apps where control matters.

ruby
class ProductDecorator < ApplicationDecorator
  delegate :id, :name, :price, :created_at, :persisted?

  def formatted_price
    h.number_to_currency(price)
  end
end

Accessing the Wrapped Object

Three equivalent ways to access the model:

ruby
class ArticleDecorator < ApplicationDecorator
  delegate_all

  def display_title
    object.title.upcase      # via 'object'
    model.title.upcase       # via 'model' (alias)
    article.title.upcase     # via model name (auto-generated)
  end
end

Accessing Rails Helpers

Use h or helpers to access view helpers:

ruby
class PostDecorator < ApplicationDecorator
  delegate_all

  def formatted_body
    h.simple_format(body)
  end

  def edit_link
    h.link_to("Edit", h.edit_post_path(object), class: "btn")
  end

  def publication_date
    h.l(published_at, format: :long)  # l is localize alias
  end
end

Decorating in Controllers

Decorate at the last moment, right before rendering:

ruby
class PostsController < ApplicationController
  def show
    @post = Post.find(params[:id]).decorate
  end

  def index
    @posts = Post.includes(:author).all.decorate
  end
end

Critical: Always use includes BEFORE decorating to avoid N+1 queries.

Association Decoration

Use decorates_association to auto-decorate associations:

ruby
class PostDecorator < ApplicationDecorator
  delegate_all
  decorates_association :author
  decorates_association :comments
  decorates_association :recent_comments, scope: :recent
end

In views, @post.author returns AuthorDecorator, not Author.

Context Passing

Pass extra data to decorators via context:

ruby
# Controller
@product = Product.find(params[:id]).decorate(context: { current_user: })

# Decorator
class ProductDecorator < ApplicationDecorator
  delegate_all

  def admin_price_info
    return unless context[:current_user]&.admin?
    "Cost: #{h.number_to_currency(cost)} | Margin: #{margin}%"
  end
end

Collection Decoration

ruby
# Auto-infers decorator from model
@products = Product.all.decorate

# Explicit decorator
@products = ProductDecorator.decorate_collection(Product.all)

# With pagination (use custom collection decorator)
class PaginatingDecorator < Draper::CollectionDecorator
  delegate :current_page, :total_pages, :limit_value
end

class ProductDecorator < ApplicationDecorator
  def self.collection_decorator_class
    PaginatingDecorator
  end
end

Testing Decorators

Place specs in spec/decorators/. Draper auto-configures RSpec integration.

Basic Pattern

ruby
# spec/decorators/user_decorator_spec.rb
require 'rails_helper'

RSpec.describe UserDecorator do
  subject(:decorator) { described_class.new(user) }

  let(:user) { build_stubbed(:user, first_name: "John", last_name: "Doe") }

  describe "#full_name" do
    subject(:full_name) { decorator.full_name }

    it "combines first and last name" do
      expect(full_name).to eq("John Doe")
    end
  end

  describe "#formatted_created_at" do
    subject(:formatted_date) { decorator.formatted_created_at }

    let(:user) { build_stubbed(:user, created_at: Time.zone.parse("2024-01-15")) }

    it "formats date in long format" do
      expect(formatted_date).to eq("January 15, 2024")
    end
  end
end

Testing with Helpers

Access helpers via helpers method in tests:

ruby
RSpec.describe PostDecorator do
  subject(:decorator) { described_class.new(post) }

  let(:post) { create(:post) }

  it "generates correct path" do
    expect(decorator.edit_link).to include(helpers.edit_post_path(post))
  end
end

Testing HTML Output with Capybara

ruby
RSpec.describe StatusDecorator do
  subject(:decorator) { described_class.new(order) }

  describe "#status_badge" do
    subject(:badge) { decorator.status_badge }

    context "when completed" do
      let(:order) { build_stubbed(:order, :completed) }

      it "renders success badge" do
        markup = Capybara.string(badge)
        expect(markup).to have_css("span.badge-success", text: "Completed")
      end
    end
  end
end

Common Anti-Patterns

Fat Decorator

Split large decorators into context-specific ones:

ruby
# Instead of one 500-line UserDecorator, use:
class Users::ProfileDecorator < ApplicationDecorator
  # Profile-related presentation
end

class Users::AdminDecorator < ApplicationDecorator
  # Admin panel presentation
end

N+1 Queries

ruby
# BAD - triggers N+1
@posts = Post.all.decorate
# In decorator: author.name triggers query per post

# GOOD - eager load first
@posts = Post.includes(:author).all.decorate

Decorating Too Early

ruby
# BAD - decorated objects in business logic
def publish(decorated_post)
  decorated_post.update(published: true)
end

# GOOD - use models for business logic
def publish(post)
  post.update(published: true)
end
# Decorate only in controller before render

Using Decorators in Models

ruby
# BAD - model references decorator
class Post < ApplicationRecord
  def display_title
    PostDecorator.new(self).formatted_title
  end
end

# GOOD - keep models unaware of decorators

Quick Reference

Method Purpose
object / model Access wrapped object
h / helpers Access view helpers
context Access passed context hash
delegate_all Delegate all methods to object
decorates_association Auto-decorate associations
decorate Decorate single object
decorate_collection Decorate collection

Additional Resources

Reference Files

For detailed patterns and examples:

  • references/patterns.md - Advanced patterns, association decoration, context handling
  • references/testing.md - Comprehensive RSpec testing guide
  • references/anti-patterns.md - Detailed anti-patterns with solutions

Example Files

Working examples in examples/:

  • examples/application_decorator.rb - Base decorator template
  • examples/model_decorator.rb - Full decorator example
  • examples/decorator_spec.rb - Complete spec template

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