Data Noise

In this article I am going to describe several ways you can debug your Rails application with ruby-debug starting from simplest and proceeding with more sophisticated setup.

Simplest of all

The simplest and most obvious way is to start your application with rdebug script:

    $ rdebug ./script/server webrick

This method doesn't work with lighttpd since the latter starts several child Rails processes.

When you execute this command you end up with debugger prompt where you can set up your breakpoints.

Debugging without rdebug script

First you need to activate the debugger in your development.rb file:

    ./config/environments/development.rb:

    ...
    require 'ruby-debug'
    Debugger.start

Now you can use Kernel#debugger method to activate debugger:

    def my_action
      debugger
      ...
    end

Remote debugging.

If for some reason you want to connect remotely to debugger, you can start it with remote debugging enabled:

    $ rdebug -s ./script/server webrick

and then connect to it with

    $ rdebug -c
    Connected.
    /usr/local/lib/ruby/1.8/webrick/server.rb:91: if svrs = IO.select(@listeners, nil, nil, 2.0)
    (rdb:1)

Once again, at this point you can add breakpoints and proceed.

Adding breakpoints directly in your code

What if you don't want to setup breakpoints all the time from the debugger's command prompt and you want to use Kernel#debugger method instead. In this case when you start a debugger, you should notify it not stop when a remote client is connected with -n option:

    $ rdebug -sn ./script/server webrick

Remote debugging without rdebug script

As always activate your debugger in development.rb file:

    ./config/environments/development.rb:

    ...
    require 'ruby-debug'
    Debugger.start_remote

Here you can control it with two options:

    ./config/environments/development.rb:

    ...
    require 'ruby-debug'
    Debugger.wait_connection = true
    Debugger.stop_on_connect = true
    Debugger.start_remote

• Debugger.wait_connection options makes debugger wait until you connect to it remotely.

• Debugger.stop_on_connect drop you to the debugger prompt as soon as you connect to it.

The method I use most of the time

All described so far methods suffer from one thing: the debugger is always activated. It can slow down your application significantly. What if I want to activate it only in a particular place in my code? As always there are two ways to do that. But first you should only require debugger in development.rb file without starting it:

    ./config/environments/development.rb:

    ...
    require 'ruby-debug'

• Debugger#start takes a block:

  def my_action
    ...
    # at this point the debugger is still disabled
    Debugger.start do
      # here debugger is enabled
      # below goes the code you want to debug
      ...
    end
    # at this point the debugger is disabled
    ...
  end
  

• Use Module#debug_method (available since version 0.4.2):

  class MyController < ApplicationController
    def my_action
      ...
    end
    debug_method :my_action
  end
  

  Now whenever you reach my_action method, the debugger is activated.