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Rails Debug Toolkit
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Rails Debug Toolkit


rails_dt gem gives you DT.p() method you can use anywhere in your project to print your debug messages.

It's somewhat similar to Ruby's native p() with output being sent to log, console and web.

For example, DT.p "Hello, world!" invoked in RootController will give you a:

[DT app/controllers/root_controller.rb:3] Hello, world!

The Ideas Behind It

  • Debug message printer must not require initialization.
  • Debug message printer must be nothing else, but a debug message printer.
  • Debug message printer must be simple and invoked always the same way regardless of where you call it from.
  • Debug message printer calls must be clearly visible in the code.
  • Debug message printer must print its location in code so you can find and modify/remove it as easy as possible.

Express Setup (Rails 3)

In your Gemfile, add:

gem "rails_dt"

Then do a bundle install.

This gives you an express (zero-conf) setup, which outputs messages to log, log/dt.log and console.

Express Setup (Rails 2)

$ gem sources --add
$ gem install rails_dt

In your config/environment.rb, add:

config.gem "rails_dt"

Setting Up Web Output (Both Rails 3 and Rails 2)

In your application root, do a:

$ rails generate rails_dt     # Rails 3
$ script/generate rails_dt    # Rails 2

Follow the instructions the generator gives you then. They are listed below.

Inside your ApplicationController class, add:


Inside your app/views/layouts/application.html.erb <head> section, add:

<%= stylesheet_link_tag "dt" %>

Inside your app/views/layouts/application.html.erb <body> section, add:

<div class="DT">
  <%= DT.web_messages_as_html %>

Checking Setup

Somewhere in your app/views/layouts/application.html.erb, add:

<% DT.p "hello, world!" %>

Refresh the page. You should see "hello, world!":

  • In your application log.
  • In log/dt.log.
  • On the web page, if you've set it up (see above).



def before_save
  DT.p "in before_save"


def action
  DT.p "hi, I'm #{action_name}"


<div class="body">
  <% DT.p "@users", @users %>


Insert debugging code:

before_filter do
  DT.p "in before_filter xyz"

after_filter do
  DT.p "in after_filter xyz"

See it in action:

$ tail -f log/dt.log


Just use DT.p anywhere you want.

Customizing Output Format

Create a sample initializer, by doing a:

$ rails generate rails_dt     # Rails 3
$ script/generate rails_dt    # Rails 2

In config/initializers/dt.rb you'll see something like:

# Customize your DT output.
#DT.log_prefix = "[DT <%= file_rel %>:<%= line %>] "
#DT.console_prefix = "[DT <%= file_rel %>:<%= line %>] "
#DT.web_prefix = '<a href="txmt://open?url=file://<%= file %>&line=<%= line %>"><%= file_rel %>:<%= line %></a> '

Uncomment and edit lines appropriately. Restart server for changes to take effect.

Values are in ERB format. The following macros are available:

  • file -- full path to file.
  • file_base -- file base name.
  • file_rel -- file name relative to Rails application root.
  • line -- line number.

You can also disable particular output target by setting its prefix to nil:

DT.console_prefix = nil     # Disable console output.


Send bug reports, suggestions and criticisms through project's page on GitHub.

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