Implementation Safety Skill

Comprehensive safety checklists to prevent common Rails bugs and vulnerabilities during implementation.

Quick Reference

Use these checklists before marking any file complete during Phase 4 (Implementation).

1. Nil Safety Checklist

Prevent NoMethodError: undefined method for nil errors.

# BAD - Crashes if user is nil
user.email.downcase

# GOOD - Safe navigation
user&.email&.downcase

# BAD - Crashes if find_by returns nil
User.find_by(email: email).name

# GOOD - Handle nil explicitly
User.find_by(email: email)&.name || "Unknown"

Checklist:

• Use safe navigation (&.) for potentially nil objects
• Add presence validations for required attributes
• Handle nil cases explicitly in conditionals
• Use find_by! or handle find_by returning nil
• Check for nil before calling methods
• Filter nil values from collections: hash.compact.each

2. ActiveRecord Safety Checklist

Prevent N+1 queries, validation failures, and data integrity issues.

# BAD - N+1 queries
Post.all.each { |p| puts p.author.name }

# GOOD - Eager loading
Post.includes(:author).each { |p| puts p.author.name }

# BAD - Silent validation failure
user.save

# GOOD - Handle validation explicitly
if user.save
  redirect_to user
else
  render :edit, status: :unprocessable_entity
end

Checklist:

• Use includes/joins to prevent N+1 queries
• Add validations for all user inputs
• Handle validation failures explicitly
• Add indexes on foreign keys
• Use scopes instead of class methods for queries
• Add counter caches for frequently accessed counts

3. Security Checklist

Prevent SQL injection, XSS, mass assignment, and other vulnerabilities.

# BAD - SQL injection
User.where("email = '#{params[:email]}'")

# GOOD - Parameterized query
User.where(email: params[:email])
User.where("email = ?", params[:email])

# BAD - Mass assignment vulnerability
User.create(params[:user])

# GOOD - Strong parameters
User.create(user_params)

private
def user_params
  params.require(:user).permit(:name, :email)
end

Checklist:

• Strong parameters for all user inputs
• No string interpolation in SQL queries
• Sanitize HTML output (sanitize or strip_tags)
• No mass assignment without whitelisting
• Use has_secure_password for authentication
• No sensitive data in logs or error messages

4. Error Handling Checklist

Prevent crashes, ensure proper logging, and return meaningful errors.

# BAD - Catches everything, hides bugs
rescue => e
  render json: { error: e.message }

# GOOD - Specific exception handling
rescue ActiveRecord::RecordNotFound => e
  render json: { error: "Resource not found" }, status: :not_found
rescue ActiveRecord::RecordInvalid => e
  render json: { errors: e.record.errors.full_messages }, status: :unprocessable_entity

Checklist:

• Rescue specific exceptions, not StandardError
• Log errors with context: Rails.logger.error("Context: #{e.message}")
• Return meaningful error messages (not raw exceptions)
• Handle edge cases (empty arrays, nil values, zero amounts)
• Use Result pattern for service objects

5. Performance Checklist

Prevent slow queries, memory bloat, and inefficient operations.

# BAD - Loads all records into memory
User.all.map(&:email)

# GOOD - Only fetches emails
User.pluck(:email)

# BAD - Counts by loading records
User.all.any?

# GOOD - Uses SQL EXISTS
User.exists?

# BAD - Loads all records at once
User.all.each { |u| process(u) }

# GOOD - Batches of 1000
User.find_each { |u| process(u) }

Checklist:

• Use pluck/select for specific columns
• Use exists? instead of any? or count > 0
• Use find_each for large collections
• Add database indexes for frequently queried columns
• Use counter caches instead of repeated count queries

6. Migration Safety Checklist

Prevent data loss and ensure reversible migrations.

# GOOD - Complete migration with all safety measures
class CreateOrders < ActiveRecord::Migration[7.1]
  def change
    create_table :orders do |t|
      t.references :user, null: false, foreign_key: true, index: true
      t.string :status, null: false, default: 'pending'
      t.decimal :total, precision: 10, scale: 2, null: false

      t.timestamps
    end

    add_index :orders, :status
    add_index :orders, [:user_id, :status]
  end
end

Checklist:

• Add indexes on all foreign keys
• Include null: false for required columns
• Add unique indexes for uniqueness constraints
• Make migrations reversible (provide down method if needed)
• Add default values where appropriate
• Use precision/scale for decimal columns

7. Specific Error Prevention

NoMethodError Prevention

# Pattern: Safe navigation chain
result = object&.method1&.method2&.method3

# Pattern: Filter nil from collections
hash.compact.each do |key, value|
  # key and value are guaranteed non-nil
end

# Pattern: Explicit nil checks
if value.nil?
  handle_missing_value
else
  process(value)
end

N+1 Query Prevention

# Pattern: Preload in controller
def index
  @posts = Post.includes(:author, :comments, :tags)
end

# Pattern: Counter cache in model
class Post < ApplicationRecord
  belongs_to :author, counter_cache: true
end

# Pattern: Test with Bullet gem
# config/environments/development.rb
Bullet.enable = true
Bullet.rails_logger = true

Security Vulnerability Prevention

# Pattern: Define strong parameters for every action
class PostsController < ApplicationController
  private

  def post_params
    params.require(:post).permit(:title, :body, :published_at)
  end

  def filter_params
    params.permit(:status, :author_id, :created_after)
  end
end

# Pattern: Parameterized queries only
User.where("email LIKE ?", "%#{sanitized_input}%")
User.where(status: params[:status])

8. Integration with Rails Error Prevention Skill

This skill provides quick checklists. For detailed patterns, examples, and edge cases, reference the rails-error-prevention skill:

# Discover full error prevention patterns
cat .claude/skills/rails-error-prevention/SKILL.md

Cross-reference:

• Nil Safety → rails-error-prevention Section 2
• ActiveRecord Safety → rails-error-prevention Section 3
• Security → rails-error-prevention Section 4
• Error Handling → rails-error-prevention Section 5

Usage in Implementation Phase

During Phase 4 (Implementation), before marking each file complete:

1. Read this skill for quick checklist reference
2. Review the file against each applicable checklist
3. Fix any violations before proceeding
4. Mark file complete only after all checks pass

# Implementation workflow
# 1. Generate code for file
# 2. Run implementation-safety checklist
# 3. Fix any issues found
# 4. Run tests
# 5. Mark file complete