---

name: dhh-coder description: Write Ruby and Rails code in DHH's distinctive 37signals style. Use this skill when writing Ruby code, Rails applications, creating models, controllers, or any Ruby file. Triggers on Ruby/Rails code generation, refactoring requests, or when the user mentions DHH, 37signals, Basecamp, HEY, Fizzy, or Campfire style.

DHH Ruby/Rails Style Guide

Write Ruby and Rails code following DHH's philosophy: clarity over cleverness, convention over configuration, developer happiness above all.

Quick Reference

Controller Actions

• Only 7 REST actions: index, show, new, create, edit, update, destroy
• New behavior? Create a new controller, not a custom action
• Action length: 1-5 lines maximum
• Empty actions are fine: Let Rails convention handle rendering

class MessagesController < ApplicationController
  before_action :set_message, only: %i[ show edit update destroy ]

  def index
    @messages = @room.messages.with_creator.last_page
    fresh_when @messages
  end

  def show
  end

  def create
    @message = @room.messages.create_with_attachment!(message_params)
    @message.broadcast_create
  end

  private
    def set_message
      @message = @room.messages.find(params[:id])
    end

    def message_params
      params.require(:message).permit(:body, :attachment)
    end
end

Private Method Indentation

Indent private methods one level under private keyword:

  private
    def set_message
      @message = Message.find(params[:id])
    end

    def message_params
      params.require(:message).permit(:body)
    end

Model Design (Fat Models)

Models own business logic, authorization, and broadcasting:

class Message < ApplicationRecord
  belongs_to :room
  belongs_to :creator, class_name: "User"
  has_many :mentions

  scope :with_creator, -> { includes(:creator) }
  scope :page_before, ->(cursor) { where("id < ?", cursor.id).order(id: :desc).limit(50) }

  def broadcast_create
    broadcast_append_to room, :messages, target: "messages"
  end

  def mentionees
    mentions.includes(:user).map(&:user)
  end
end

class User < ApplicationRecord
  def can_administer?(message)
    message.creator == self || admin?
  end
end

Current Attributes

Use Current for request context, never pass current_user everywhere:

class Current < ActiveSupport::CurrentAttributes
  attribute :user, :session
end

# Usage anywhere in app
Current.user.can_administer?(@message)

Ruby Syntax Preferences

# Symbol arrays with spaces inside brackets
before_action :set_message, only: %i[ show edit update destroy ]

# Modern hash syntax exclusively
params.require(:message).permit(:body, :attachment)

# Single-line blocks with braces
users.each { |user| user.notify }

# Ternaries for simple conditionals
@room.direct? ? @room.users : @message.mentionees

# Bang methods for fail-fast
@message = Message.create!(params)
@message.update!(message_params)

# Predicate methods with question marks
@room.direct?
user.can_administer?(@message)
@messages.any?

# Expression-less case for cleaner conditionals
case
when params[:before].present?
  @room.messages.page_before(params[:before])
when params[:after].present?
  @room.messages.page_after(params[:after])
else
  @room.messages.last_page
end

Query Optimization

# WRONG: Load all records then extract attribute
users.map(&:name)

# CORRECT: Pluck directly from database
users.pluck(:name)

# WRONG: Count via Ruby
messages.to_a.count

# CORRECT: Count via SQL
messages.count

StringInquirer for Predicates

Use .inquiry on string enums for readable conditionals:

class Event < ApplicationRecord
  def action
    super.inquiry
  end
end

# Clean predicate methods
event.action.completed?
event.action.pending?
event.action.failed?

Controller Response Patterns

# Return 204 No Content for successful updates without body
def update
  @message.update!(message_params)
  head :no_content
end

# Return 201 Created for successful creates
def create
  @message = Message.create!(message_params)
  head :created
end

My:: Namespace for Current User Resources

Use My:: namespace for resources scoped to Current.user:

# routes.rb
namespace :my do
  resource :profile, only: %i[ show edit update ]
  resources :notifications, only: %i[ index destroy ]
end

# app/controllers/my/profiles_controller.rb
class My::ProfilesController < ApplicationController
  def show
    @profile = Current.user
  end
end

No index or show with ID needed—resource is implicit from Current.user.

Compute at Write Time

Perform data manipulation during saves, not during presentation:

# WRONG: Compute on read
def display_name
  "#{first_name} #{last_name}".titleize
end

# CORRECT: Compute on write
before_save :set_display_name

private
  def set_display_name
    self.display_name = "#{first_name} #{last_name}".titleize
  end

Benefits: enables pagination, caching, and reduces view complexity.

Delegate for Lazy Loading

Use delegate to enable lazy loading through associations:

class Message < ApplicationRecord
  belongs_to :session
  delegate :user, to: :session
end

# Lazy loads user through session
message.user

Naming Conventions

Element	Convention	Example
Setter methods	set_ prefix	set_message, set_room
Parameter methods	{model}_params	message_params
Association names	Semantic, not generic	creator not user
Scopes	Chainable, descriptive	with_creator, page_before
Predicates	End with ?	direct?, can_administer?
Current user resources	My:: namespace	My::ProfilesController

Hotwire/Turbo Patterns

Broadcasting is model responsibility:

# In model
def broadcast_create
  broadcast_append_to room, :messages, target: "messages"
end

# In controller
@message.broadcast_replace_to @room, :messages,
  target: [ @message, :presentation ],
  partial: "messages/presentation",
  attributes: { maintain_scroll: true }

Error Handling

Rescue specific exceptions, fail fast with bang methods:

def create
  @message = @room.messages.create_with_attachment!(message_params)
  @message.broadcast_create
rescue ActiveRecord::RecordNotFound
  render action: :room_not_found
end

State as Records (Not Booleans)

Track state via database records rather than boolean columns:

# WRONG: Boolean columns for state
class Card < ApplicationRecord
  # closed: boolean, gilded: boolean columns
end
card.update!(closed: true)
card.closed?  # Loses who/when/why

# CORRECT: State as separate records
class Card < ApplicationRecord
  has_one :closure
  has_one :gilding

  def close(by:)
    create_closure!(closed_by: by)
  end

  def closed?
    closure.present?
  end
end
card.close(by: Current.user)
card.closure.closed_by  # Full audit trail

REST URL Transformations

Map custom actions to nested resource controllers:

Custom Action	REST Resource
POST /cards/:id/close	POST /cards/:id/closure
DELETE /cards/:id/close	DELETE /cards/:id/closure
POST /cards/:id/gild	POST /cards/:id/gilding
POST /posts/:id/publish	POST /posts/:id/publication
DELETE /posts/:id/publish	DELETE /posts/:id/publication

# routes.rb
resources :cards do
  resource :closure, only: %i[ create destroy ]
  resource :gilding, only: %i[ create destroy ]
end

# app/controllers/cards/closures_controller.rb
class Cards::ClosuresController < ApplicationController
  def create
    @card = Card.find(params[:card_id])
    @card.close(by: Current.user)
  end

  def destroy
    @card = Card.find(params[:card_id])
    @card.closure.destroy!
  end
end

Architecture Preferences

Traditional	DHH Way
PostgreSQL	SQLite (for single-tenant)
Redis + Sidekiq	Solid Queue
Redis cache	Solid Cache
Kubernetes	Single Docker container
Service objects	Fat models
Policy objects (Pundit)	Authorization on User model
FactoryBot	Fixtures
Boolean state columns	State as records

Detailed References

For comprehensive patterns and examples, see:

Core Patterns

• references/patterns.md - Complete code patterns with explanations
• references/palkan-patterns.md - Namespaced model classes, counter caches, model organization order, PostgreSQL enums
• references/concerns-organization.md - Model-specific vs common concerns, facade pattern
• references/delegated-types.md - Polymorphism without STI problems
• references/recording-pattern.md - Unifying abstraction for diverse content types
• references/filter-objects.md - PORO filter objects, URL-based state, testable query building
• references/database-patterns.md - UUIDv7, hard deletes, state as records, counter caches, indexing

Rails Components

• references/activerecord-tips.md - ActiveRecord query patterns, validations, associations
• references/controllers-tips.md - Controller patterns, routing, rate limiting, form objects
• references/hotwire-tips.md - Turbo Frames, Turbo Streams, Stimulus, ViewComponents
• references/turbo-morphing.md - Turbo 8 page refresh with morphing patterns
• references/activestorage-tips.md - File uploads, attachments, blob handling
• references/stimulus-catalog.md - Copy-paste-ready Stimulus controllers (clipboard, dialog, hotkey, etc.)

Frontend

• references/css-architecture.md - Native CSS patterns (layers, OKLCH, nesting, dark mode)

Authentication & Multi-Tenancy

• references/passwordless-auth.md - Magic link authentication, sessions, identity model
• references/multi-tenancy.md - Path-based tenancy, cookie scoping, tenant-aware jobs

Infrastructure & Integrations

• references/webhooks.md - Secure webhook delivery, SSRF protection, retry strategies
• references/caching-strategies.md - Russian Doll caching, Solid Cache, cache analysis
• references/config-tips.md - Configuration, logging, deployment patterns
• references/structured-events.md - Rails 8.1 Rails.event API for structured observability
• references/resources.md - Links to source material and further reading

Philosophy Summary

1. REST purity: 7 actions only; new controllers for variations
2. Fat models: Authorization, broadcasting, business logic in models
3. Thin controllers: 1-5 line actions; extract complexity
4. Convention over configuration: Empty methods, implicit rendering
5. Minimal abstractions: No service objects for simple cases
6. Current attributes: Thread-local request context everywhere
7. Hotwire-first: Model-level broadcasting, Turbo Streams, Stimulus
8. Readable code: Semantic naming, small methods, no comments needed

Success Indicators

Code aligns with DHH style when:

• Controllers map CRUD verbs to resources (no custom actions)
• Models use concerns for horizontal behavior sharing
• State uses records instead of boolean columns
• Abstractions remain minimal (no unnecessary service objects)
• Database backs solutions (Solid Queue/Cache, not Redis)
• Turbo/Stimulus handle all interactivity
• Authorization lives on User model (can_*? methods)
• Current attributes provide request context
• Scopes follow naming conventions (chronologically, with_*, etc.)
• Uses pluck over map for attribute extraction
• Current user resources use My:: namespace
• Data computed at write time, not presentation