Decorate With Parts

decorate-with-parts

Use this skill when creating View Parts for decorator-style logic in Hanami 2.x.

Core principle: Parts wrap data with presentation methods. They keep Views and templates free of complex formatting logic.

Quick Reference

Core Rules

1. Create the Part class:

   # app/views/parts/user.rb
   # frozen_string_literal: true
   
   module MyApp
     module Views
       module Parts
         class User < Hanami::View::Part
           delegate :name, :email, to: :value
   
           def display_name
             "#{value.first_name} #{value.last_name}"
           end
   
           def member_since
             value.created_at.strftime("%B %Y")
           end
   
           def admin?
             value.role == "admin"
           end
         end
       end
     end
   end
   

2. Expose data as a Part in the View:

   class Show < MyApp::View
     expose :user, as: :user_part
   end
   

3. Use Part methods in templates:

   <h1><%= user_part.display_name %></h1>
   <p>Member since <%= user_part.member_since %></p>
   
   <% if user_part.admin? %>
     <span class="badge">Admin</span>
   <% end %>
   

4. Keep Parts focused on presentation. No database queries, no business rules:

   # GOOD: formatting and simple predicates
   def display_name
     "#{value.first_name} #{value.last_name}"
   end
   
   # BAD: business logic
   def can_delete?(resource)
     value.role == "admin" && resource.owner_id == value.id
   end
   

5. Delegate common methods to the wrapped value:

   delegate :id, :name, :email, :created_at, to: :value
   

6. Access the raw value with the value method when needed:

   def raw_attributes
     value.to_h
   end
   

7. Do not generate HTML in Parts. Parts return strings or booleans. Templates handle HTML:

   # GOOD
   def status_label
     value.active? ? "Active" : "Inactive"
   end
   
   # BAD
   def status_badge
     "<span class='badge'>#{status_label}</span>"
   end
   

Common Mistakes

• No database queries in Parts. Parts wrap already-fetched data; queries belong in repositories or actions.
• No HTML generation in Parts. Parts return plain strings or booleans; templates produce HTML markup.
• No business logic in Parts. Predicates based on simple attribute values are fine; authorization rules and domain decisions belong in interactors or service objects.

Integration