Rails Debugging Tools & Techniques
<superpowers-integration> REQUIRED BACKGROUND: Use superpowers:systematic-debugging for investigation process
- That skill defines 4-phase framework (Root Cause → Pattern → Hypothesis → Implementation)
- This skill provides Rails-specific debugging tools for each phase
</superpowers-integration>
<when-to-use>
- Rails application behaving unexpectedly
- Tests failing with unclear errors
- Performance issues or N+1 queries
- Production errors need investigation
</when-to-use>
<verification-checklist> Before completing debugging work:
- ✅ Root cause identified (not just symptoms)
- ✅ Regression test added (prevents recurrence)
- ✅ Fix verified in development and test environments
- ✅ All tests passing (bin/ci passes)
- ✅ Logs reviewed for related issues
- ✅ Performance impact verified (if applicable)
</verification-checklist>
<phase1-root-cause-investigation>
<tool name="rails-logs"> <description>Check Rails logs for errors and request traces</description>
# 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
</tool>
<tool name="rails-console"> <description>Interactive Rails console for testing models/queries</description>
# 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
</tool>
<tool name="byebug"> <description>Breakpoint debugger for stepping through code</description>
# 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
</tool>
<tool name="sql-logging"> <description>Enable verbose SQL logging to see queries</description>
# In rails console or code
ActiveRecord::Base.logger = Logger.new(STDOUT)
# Now all SQL queries print to console
User.all
# => SELECT "users".* FROM "users"
</tool>
</phase1-root-cause-investigation>
<phase2-pattern-analysis>
<tool name="rails-routes"> <description>Check route definitions and paths</description>
# List all routes
rails routes
# Filter routes
rails routes | grep users
# Show routes for controller
rails routes -c users
</tool>
<tool name="rails-db-status"> <description>Check migration status and schema</description>
# Migration status
rails db:migrate:status
# Show schema version
rails db:version
# Check pending migrations
rails db:abort_if_pending_migrations
</tool>
</phase2-pattern-analysis>
<phase3-hypothesis-testing>
<tool name="rails-runner"> <description>Run Ruby code in Rails environment</description>
# 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)"
</tool>
</phase3-hypothesis-testing>
<phase4-implementation>
<tool name="rails-test-verbose"> <description>Run tests with detailed output</description>
# 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
</tool>
</phase4-implementation>
<common-issues>
<issue name="n-plus-one-queries"> <detection> Check logs for many similar queries:
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
</detection> <solution> Use includes/preload:
# Bad
users.each { |user| user.posts.count }
# Good
users.includes(:posts).each { |user| user.posts.count }
</solution> </issue>
<issue name="missing-migration"> <detection> Error: "ActiveRecord::StatementInvalid: no such column" </detection> <solution>
# Check migration status
rails db:migrate:status
# Run pending migrations
rails db:migrate
# Or rollback and retry
rails db:rollback
rails db:migrate
</solution> </issue>
</common-issues>
<related-skills>
- superpowers:systematic-debugging (4-phase framework)
- rails-ai:models (Query optimization, N+1 debugging)
- rails-ai:controllers (Request debugging, parameter inspection)
- rails-ai:testing (Test debugging, failure investigation)
</related-skills>
<resources>
Official Documentation:
Gems & Libraries:
Tools:
- Rack Mini Profiler - Performance profiling
</resources>