Debugging Rails

Michael DenomyBoston Ruby Project NightMarch 5, 2013

Agenda

•Introduction to Rails Debugging

-Role of testing

-Examining the log files

- Interpreting the stack trace

-The better_errors gem

-Rails console

-Using pry gem to debug

Testing• First line of defense

- Avoid defects in the first place

• Since adopting TDD, time spent ‣ using a debugger

‣ writing log messages

‣ triaging defects

have all dropped significantly

• Find a defect?

- First step: write a test to reproduce it

General Debugging Tips• Reproduce the problem

- Under what conditions does it occur

‣ data, configuration, environment

- Write a unit/integration test if possible

• Divide and Conquer

- Reduce the places you have to look

‣ Logging statements

‣ Simplest conditions to reproduce

‣ Any recent changes?

General Debugging Tips• Pay attention to the data

- Don’t jump to conclusions

- Avoid “that can’t be the problem”

• Be methodical

- Talk the problem out

‣ Use “rubber ducky debugging” if solo

- Change one thing at a time

- Keep track of changes/ideas you’ve tried out

- Periodically circle back and re-evaluate

Rails Log Files• Rails keeps log file for each environment

- e.g. development.log, production.log

• Log messages have severity levels

- :debug, :info, :warn, :error, :fatal

- config.log.level filters what gets logged

• Writing your own log messages

Asset Pipeline Log Messages

• Lots of messages related to asset pipeline

• quiet_assets gem

- suppresses asset pipeline messages

- should be used in development only

• thin web server

- fixes a problem with message that shows up with quiet_assets gem and WEBrick

• Gemfile

Reading A Stack Trace

The better_errors gem• Richer error page

‣ Full stack trace

‣ Source code inspection for all stack frames

‣ Local and instance variable inspection

‣ Live REPL on every stack frame

• Advanced features requires ‘binding_of_caller’ gem

• Gemfile - Development only!

Using the Rails Console• Great playground to experiment

- Especially handy for model access

• Danger!!!

- Good idea to run in sandbox to prevent changing data

rails console --sandbox

pry gem• pry is an IRB replacement

- live help

- syntax highlighting

- navigate scope

- and much, much, more

• pry also supports plugins...lots of them

- check out www.rubygems.org for full list

• pry-debugger gem

- step, next, continue

Debug Using an IDE• If you come from a background where

you have used an IDE, e.g. Java or C#, an IDE is an option

• RubyMine has an integrated debugger

- Does not play well with pry-debug or other debuggers

Summary• Divide and Conquer Strategy

• Be methodical

• Check the logs for clues

• Testing is the best defense

• Questions?

Referenceshttp://gipsysoft.com/articles/debug-rules/

http://pryrepl.org/screencasts.html

http://railscasts.com/episodes/86-logging-variables

http://railscasts.com/episodes/24-the-stack-trace

http://railscasts.com/episodes/280-pry-with-rails

http://samuelmullen.com/2012/07/getting-more-out-of-the-rails-console/

Debugging Rails

Michael DenomyBoston Ruby Project NightMarch 5, 2013