Sandi Metz Rules

Sandi Metz Rules

Overview

This skill helps apply Sandi Metz's four rules for writing maintainable object-oriented code to Ruby codebases. These rules are heuristics that encourage good design practices, making code easier to understand, test, and maintain.

The Four Rules

1. Classes can be no longer than 100 lines of code
2. Methods can be no longer than 5 lines of code
3. Pass no more than 4 parameters into a method
4. Controllers can instantiate only one object

When to Use This Skill

Apply this skill when:

• Users explicitly request applying Sandi Metz's rules
• Reviewing Ruby code for maintainability and code quality
• Refactoring existing Ruby code to improve design
• Users ask about "code smells" or improving object-oriented design
• Analyzing Ruby code for complexity or maintainability issues
• Users mention terms like "POODR" or reference Sandi Metz's work

Code Review Workflow

1. Analyze the Code

When reviewing code against Sandi Metz's rules:

Read the reference document: First, load the detailed rules documentation:

Read references/rules.md

Measure accurately:

• For classes: Count lines of actual code (exclude blank lines, comments, class definition, and end statements)
• For methods: Count lines of actual code (exclude blank lines, comments, method definition, and end statements)
• For parameters: Count all explicit parameters including keyword arguments (but not &block)
• For controllers: Count object instantiations in each action (excluding finding existing objects)