Rails Debugging

Rails Debugging Tools & Techniques

# Development logs
tail -f log/development.log

# Production logs (Kamal)
kamal app logs --tail

# Filter by severity
grep ERROR log/production.log

# Filter by request
grep "Started GET" log/development.log

# Start console
rails console

# Or production console (Kamal)
kamal app exec 'bin/rails console'

# Test models
user = User.find(1)
user.valid?  # Check validations
user.errors.full_messages  # See errors

# Test queries
User.where(email: "test@example.com").to_sql  # See SQL
User.includes(:posts).where(posts: { published: true })  # Avoid N+1

# Add to any Rails file
def some_method
  byebug  # Execution stops here
  # ... rest of method
end

# Byebug commands:
# n  - next line
# s  - step into method
# c  - continue execution
# pp variable  - pretty print
# var local  - show local variables
# exit  - quit debugger

# In rails console or code
ActiveRecord::Base.logger = Logger.new(STDOUT)

# Now all SQL queries print to console
User.all
# => SELECT "users".* FROM "users"

# List all routes
rails routes

# Filter routes
rails routes | grep users

# Show routes for controller
rails routes -c users

# Migration status
rails db:migrate:status

# Show schema version
rails db:version

# Check pending migrations
rails db:abort_if_pending_migrations

# Run one-liner
rails runner "puts User.count"

# Run script
rails runner scripts/investigate_users.rb

# Production environment
RAILS_ENV=production rails runner "User.pluck(:email)"

# Run single test with backtrace
rails test test/models/user_test.rb --verbose

# Run with warnings enabled
RUBYOPT=-W rails test

# Run with seed for reproducibility
rails test --seed 12345

User Load (0.1ms)  SELECT * FROM users WHERE id = 1
Post Load (0.1ms)  SELECT * FROM posts WHERE user_id = 1
Post Load (0.1ms)  SELECT * FROM posts WHERE user_id = 2
Post Load (0.1ms)  SELECT * FROM posts WHERE user_id = 3

# Bad
users.each { |user| user.posts.count }

# Good
users.includes(:posts).each { |user| user.posts.count }

# Check migration status
rails db:migrate:status

# Run pending migrations
rails db:migrate

# Or rollback and retry
rails db:rollback
rails db:migrate

Official Documentation:

• Rails Guides - Debugging Rails Applications
• Rails API - ActiveSupport::Logger
• Ruby Debugging Guide

Gems & Libraries:

• byebug - Ruby debugger
• bullet - N+1 query detection

Tools:

• Rack Mini Profiler - Performance profiling