# Ruby Coder

> Use when creating new Ruby files, writing Ruby methods, or refactoring Ruby code to ensure adherence to clarity, simplicity, and maintainability standards.

- Skill: `majesticlabs-dev/ruby-coder` (Agent Skill, multi-file: 4 files)
- Install (CLI): `npx skillmds@latest add majesticlabs-dev/ruby-coder`
- Raw SKILL.md: https://api.skillmd.com/api/skills/majesticlabs-dev/ruby-coder/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: majesticlabs-dev (https://skillmd.com/u/majesticlabs-dev)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/majesticlabs-dev/ruby-coder

---


# Ruby Coder

## Ruby 3.x Modern Syntax

Use hash shorthand when keys match variable names:

```ruby
age = 49
name = "David"
user = { name:, age: }
```

For general naming conventions, semantic methods, and enumerable patterns, see `references/ruby-style-conventions.md`.

## Sandi Metz's 4 Rules for Developers

These rules enforce strict limits to maintain code quality. Breaking them requires explicit justification.

### Rule 1: Classes Can Be No Longer Than 100 Lines

**Limit**: Maximum 100 lines of code per class

**Check**: When a class exceeds this limit, extract secondary concerns to new classes

```ruby
# When exceeding 100 lines, extract secondary concerns:
class UserProfilePresenter  # Presentation logic
class UserProfileValidator  # Validation logic
class UserProfileNotifier   # Notification logic
```

**Exceptions**: Valid Single Responsibility Principle (SRP) justification required

### Rule 2: Methods Can Be No Longer Than 5 Lines

**Limit**: Maximum 5 lines per method. Each if/else branch counts as lines.

```ruby
# Good - 5 lines or fewer
def process_order
  validate_order
  calculate_totals
  apply_discounts
  finalize_payment
end

# Avoid - extract when too long
def process_order
  return unless items.any?
  return unless valid_address?
  self.subtotal = items.sum(&:price)
  self.tax = subtotal * tax_rate
  self.total = subtotal + tax
  charge_customer
  send_confirmation
end
```

**Exceptions**: Pair approval required (Rule 0: break rules with agreement)

### Rule 3: Pass No More Than 4 Parameters

**Limit**: Maximum 4 parameters per method. Use parameter objects or hashes when more data is needed.

```ruby
# Parameter object when data is related
def create_post(post_params)
  Post.create(user: post_params.user, title: post_params.title, content: post_params.content)
end
```

**Exceptions**: Rails view helpers like `link_to` or `form_for` are exempt

### Rule 4: Controllers May Instantiate Only One Object

**Limit**: Controllers should instantiate only one object; other objects come through that via facade pattern.

```ruby
# Good - single object via facade
class DashboardController < ApplicationController
  def show
    @dashboard = DashboardFacade.new(current_user)
  end
end

# Avoid - multiple instance variables
class DashboardController < ApplicationController
  def show
    @user = current_user
    @posts = @user.posts.recent
    @notifications = @user.notifications.unread
  end
end
```

- Prefix unused instance variables with underscore: `@_calculation`
- Avoid direct collaborator access in views: `@user.profile.avatar` -> use facade method

## Code Quality Standards

### Thread Safety

Use `Mutex` for shared mutable state:

```ruby
class Configuration
  @instance_mutex = Mutex.new

  def self.instance
    return @instance if @instance
    @instance_mutex.synchronize { @instance ||= new }
  end
end
```

### Idempotent Operations

Design operations to be safely repeatable:

```ruby
def activate_user
  return if user.active?
  user.update(active: true)
  send_activation_email unless email_sent?
end
```

## Refactoring Triggers

Extract classes when:
- Class exceeds 100 lines (Sandi Metz Rule 1)
- Class has multiple responsibilities
- Class name contains "And" or "Or"

Extract methods when:
- Method exceeds 5 lines (Sandi Metz Rule 2)
- Conditional logic is complex
- Code has nested loops or conditionals
- Comments explain what code does (code should be self-explanatory)

Use parameter objects when:
- Methods require more than 4 parameters (Sandi Metz Rule 3)
- Related parameters are always passed together
- Parameter list is growing over time

Create facades when:
- Controllers need multiple objects (Sandi Metz Rule 4)
- Views access nested collaborators
- Complex data aggregation is needed

## When to Break Rules

Sandi Metz's "Rule 0": Break any of the 4 rules only with pair approval or clear justification.

| Rule | Valid Exception |
|------|---------------|
| 100 lines | Clear SRP justification required |
| 5 lines | Complex but irreducible algorithms |
| 4 params | Rails view helpers exempt |
| 1 object | Simple views without facades |

Document all exceptions with clear reasoning in code comments.

## References

- `references/sandi-metz.md` - Code smells, refactoring, testing principles
- `references/ruby-tips.md` - Type conversion, hash patterns, proc composition, refinements
- `references/ruby-style-conventions.md` - Naming, semantic methods, enumerables, composition