Improving software delivery in every organisation

Troubleshooting and Debugging

1. Stacktraces

In the given stacktrace identify the most likely source of the error:

NoMethodError: undefined method `title' for nil:NilClass
  from lib/acme_limited/use_cases/all_resources.rb:10:in `block in call'
  from active_record/relation/delegation.rb:41:in `each'
  from active_record/relation/delegation.rb:41:in `each'
  from lib/acme_limited/use_cases/all_resources.rb:9:in `group_by'
  from lib/acme_limited/use_cases/all_resources.rb:9:in `call'
  from app/controllers/resource_controller.rb:12:in `index'
  from action_controller/metal/basic_implicit_render.rb:4:in `send_action'
  from abstract_controller/base.rb:186:in `process_action'
  from action_controller/metal/rendering.rb:30:in `process_action'
  from abstract_controller/callbacks.rb:20:in `block in process_action'
  from active_support/callbacks.rb:108:in `block in run_callbacks'
  from react/rails/controller_lifecycle.rb:31:in `use_react_component_helper'
  from active_support/callbacks.rb:117:in `block in run_callbacks'
  from active_support/callbacks.rb:135:in `run_callbacks'
  from abstract_controller/callbacks.rb:19:in `process_action'
  from action_controller/metal/rescue.rb:20:in `process_action'
  from action_controller/metal/instrumentation.rb:32:in `block in process_action'

Outline the process you would follow to try to resolve the above error.

2. Practice

It’s possible you will encounter legacy projects which you need to run, and may contain bugs.

In some cases, you may not be able to trust the code - naming and behaviour might not line up. A read_file method might read AND write back to a file, or read from a database instead.

Assumptions are dangerous. If the results you are getting are confusing, try to examine and investigate what you might be assuming.

Use a disciplined approach - ask yourself if you’ve made an assumption, and test those assumptions one by one.

2.1 Understand and get it running

Open “BuggyProject” up in a text editor

Read the contents of the project and understand it’s purpose. Hint: start with the tests.

Use Make to run it in the browser. Fix any bugs that prevent it from loading.

Use Make to run the tests, fix the code as you see fit.


Woah, we’ve just introduced a new tool: Make.

Run $ man make and have a read.

  • Describe to your pair what you think it does. Do you both agree?

2.2 Deliver some value

Now you have it running, and the tests are passing have a look at how it looks in the browser.

Identify issues and create a backlog of problems on post it notes.

Work through the issue you think is of the highest priority to a potential user.

3. Build deeper knowledge

What are some steps you could take to resolve errors in a gem?

Identify some methods of debugging errors in production application.