When you’re using Ruby or Rails you’ll have to use irb or rails console which is an irb session with Rails environment loaded in it.

This is an interactive console in which you can evaluate Ruby expressions and access to your project code.

Some of the tricks below will work only in Rails console but some will also work in a regular Ruby console.

Pretty printing objects

When you’re trying some code or debugging your app you’ll often need to look at the content of some variables. The default logging in IRB is not so great and readable:

>> u = User.first
#<User id: 1, email: "nico@bounga.org", encrypted_password: "$2a$10$qcytDGIYGF727KorhZJJ.Oe3gCjgVosK53IqAqJkXnmz...">

But you can have a much more readable format by using an helper that will display content by serializing it to YAML:

>> y u
--- !ruby/object:User
  id: 1
  email: nico@bounga.org
  encrypted_password: "$2a$10$qcytDGIYGF727KorhZJJ.Oe3gCjgVosK53IqAqJkXnmzNaYOPoGlO"
=> nil

I really think it’s a much more readable format especially for big objects.

Last expression value

One major issue of my brain in IRB is to forget or at least not anticipate that I’ll need the result of an evaluation available in variable to reuse it later. Luckily IRB offer a way to retrieve latest evaluation result so you can do:

>> 1 + 1
=> 2
>> res = _
=> 2
>> res
=> 2

Reload latest version of code

When working on a Rails project you’ll often find yourself using a console to test things then modify app code then want to try it again. When you do this you need to reload code between edits to benefit from it in the Rails console. To do this you can quit and start again the rails console or even better use code reloading:

>> reload!
=> true

Now you’re using the latest version of your code. Be careful, if you already stored model instances in variables you should load them again or you’ll work on older version of the objects.

Search for old commands

I often need to search for previous commands I already used that I want to evaluate again or change then evaluate. Rather than remembering everything and typing it again and again I use C-r to search for command I already evaluated and use it as a basis.


Auto-completion is a must have and a feature you should use everywhere it’s available. You should use it in your editor, in your shell but also in IRB. You can do it so take advantage of it!

Let say you’re in an IRB session and you type Acti you can then type TAB to see all available completions, choose one and complete it. It also works with methods for example:

"foo".to_ then TAB will give you something like:

>> "foo".to_
.to_c                   .to_dragonfly_unique_s  .to_json                .to_query               .to_sym
.to_d                   .to_enum                .to_json_raw            .to_r                   .to_time
.to_date                .to_f                   .to_json_raw_object     .to_s                   .to_yaml
.to_datetime            .to_i                   .to_param               .to_str                 .to_yaml_properties

Helpers are availables in Rails console

Sometimes when you’re in a Rails console you’ll want to try out some helpers you wrote earlier. You can do this through the helper method that exposes all your helpers in the console:

>> helper.number_to_currency(1.50)
=> "$1.50"

Combined with reload! it’s really useful to try your helper methods.

Requesting end-points

One other thing you’ll often want and need to try is your end-points whether they are JSON or old-good HTML ones. You could want to check if the response code is the good one, if the body contains a given string, if it redirects to something, if flash is set, …

>> app.get "/movies"
=> 200

Getting /movies was successful.

>> app.get "/users/1"
=> 302

>> app.response.redirect_url
=> "http://www.example.com/session/new"

Getting /users/1 ended in a redirection that redirects us to http://www.example.com/session/new.

>> app.post "/session", email: 'fred@example.com', password: 'secret'
=> 302

>> app.response.redirect_url
=> "http://www.example.com/users/1"

>> app.session[:user_id]
=> 1

Posting to /session results in a redirection to the user detail page and checking the session shows up that the session user_id key contains the logged in user id.

Let’s say you want to know if there’s any flash available:

>> app.flash
=> {}

You could also check for assigns in the action:

>> app.assigns[:users]
=> nil

Or even cookies:

>> app.cookies
=> {"_demo_session_id"=>"22ae45eb22ae45eb22ae45eb"}


Another wonderful thing in the Rails console is that you can start an IRB session in sanboxed mode so when you quit your session everything is rolled back. It’s especially useful in production so you can try thing being sure that everything will be in the same state as when you started your session.

$ rails console production --sandbox
Loading production environment in sandbox
Any modifications you make will be rolled back on exit

As indicated, any database changes you make will be rolled back when you exit the session.

Let’s try to delete a user:

>> User.destroy(1)

>> User.find(1)
ActiveRecord::RecordNotFound: Could not find User with id=1

>> exit
   (12.3ms)  ROLLBACK

If you start a new IRB session and try to find the user with id 1 you’ll find it as if nothing happened before. Really cool.

Searching for the sources

Another thing you’ll often need to know when you’re in an IRB session is where a given method was defined.

You can easily know this:

>> user = User.first
>> user.rating

Let’s say you want to know where the rating method is defined (even if it pretty obvious), Ruby will help you to find out. You only have to use:

>> User.instance_method(:rating).source_location
=> ["/Users/nico/demo/app/models/user.rb", 39]

You can even go further by looking at the sources of a given method by using User.instance_method(:rating).source.display.

Configuring IRB for next sessions

The last thing you need to know about IRB / Rails console is that you can configure it and be in a given state when it starts. Here is what I do in my ~/.irbrc config file:

require 'irb/completion'
require 'pp'
IRB.conf[:AUTO_INDENT] = true

  require "pry"
rescue LoadError => e
  warn "=> Unable to load pry"

In my config file I ensure completion will be available, then I require pp to be able to display objects nicely in the console to ease reading. I also make sure that IRB will indent code for me since I’m used to read indented code. Then I change IRB prompt to a simple one because I like to have a minimal prompt in IRB.

Then if Pry is available I load it.

Final words

IRB, and by extension Rails console, is a real gem and can help you a lot everyday. You really should try to take the best out of it. Maybe my next blog post will be the same king of tips but for Iex which is the same kind of tool but for Elixir




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