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Debugging Rails Applications

This guide introduces techniques for debugging Ruby on Rails applications.

After reading this guide, you will know:

1 View Helpers for Debugging

One common task is to inspect the contents of a variable. Rails provides three different ways to do this:

  • debug
  • to_yaml
  • inspect

1.1 debug

The debug helper will return a <pre> tag that renders the object using the YAML format. This will generate human-readable data from any object. For example, if you have this code in a view:

<%= debug @article %>
  <%= @article.title %>

You'll see something like this:

--- !ruby/object Article
  updated_at: 2008-09-05 22:55:47
  body: It's a very helpful guide for debugging your Rails app.
  title: Rails debugging guide
  published: t
  id: "1"
  created_at: 2008-09-05 22:55:47
attributes_cache: {}

Title: Rails debugging guide

1.2 to_yaml

Alternatively, calling to_yaml on any object converts it to YAML. You can pass this converted object into the simple_format helper method to format the output. This is how debug does its magic.

<%= simple_format @article.to_yaml %>
  <%= @article.title %>

The above code will render something like this:

--- !ruby/object Article
updated_at: 2008-09-05 22:55:47
body: It's a very helpful guide for debugging your Rails app.
title: Rails debugging guide
published: t
id: "1"
created_at: 2008-09-05 22:55:47
attributes_cache: {}

Title: Rails debugging guide

1.3 inspect

Another useful method for displaying object values is inspect, especially when working with arrays or hashes. This will print the object value as a string. For example:

<%= [1, 2, 3, 4, 5].inspect %>
  <%= @article.title %>

Will render:

[1, 2, 3, 4, 5]

Title: Rails debugging guide

2 The Logger

It can also be useful to save information to log files at runtime. Rails maintains a separate log file for each runtime environment.

2.1 What is the Logger?

Rails makes use of the ActiveSupport::Logger class to write log information. Other loggers, such as Log4r, may also be substituted.

You can specify an alternative logger in config/application.rb or any other environment file, for example:

config.logger = Logger.new(STDOUT)
config.logger = Log4r::Logger.new("Application Log")

Or in the Initializer section, add any of the following

Rails.logger = Logger.new(STDOUT)
Rails.logger = Log4r::Logger.new("Application Log")

By default, each log is created under Rails.root/log/ and the log file is named after the environment in which the application is running.

2.2 Log Levels

When something is logged, it's printed into the corresponding log if the log level of the message is equal to or higher than the configured log level. If you want to know the current log level, you can call the Rails.logger.level method.

The available log levels are: :debug, :info, :warn, :error, :fatal, and :unknown, corresponding to the log level numbers from 0 up to 5, respectively. To change the default log level, use

config.log_level = :warn # In any environment initializer, or
Rails.logger.level = 0 # at any time

This is useful when you want to log under development or staging without flooding your production log with unnecessary information.

The default Rails log level is debug in all environments.

2.3 Sending Messages

To write in the current log use the logger.(debug|info|warn|error|fatal|unknown) method from within a controller, model, or mailer:

logger.debug "Person attributes hash: #{@person.attributes.inspect}"
logger.info "Processing the request..."
logger.fatal "Terminating application, raised unrecoverable error!!!"

Here's an example of a method instrumented with extra logging:

class ArticlesController < ApplicationController
  # ...

  def create
    @article = Article.new(article_params)
    logger.debug "New article: #{@article.attributes.inspect}"
    logger.debug "Article should be valid: #{@article.valid?}"

    if @article.save
      logger.debug "The article was saved and now the user is going to be redirected..."
      redirect_to @article, notice: 'Article was successfully created.'
      render :new, status: :unprocessable_entity

  # ...

    def article_params
      params.require(:article).permit(:title, :body, :published)

Here's an example of the log generated when this controller action is executed:

Started POST "/articles" for at 2018-10-18 20:09:23 -0400
Processing by ArticlesController#create as HTML
  Parameters: {"utf8"=>"✓", "authenticity_token"=>"XLveDrKzF1SwaiNRPTaMtkrsTzedtebPPkmxEFIU0ordLjICSnXsSNfrdMa4ccyBjuGwnnEiQhEoMN6H1Gtz3A==", "article"=>{"title"=>"Debugging Rails", "body"=>"I'm learning how to print in logs.", "published"=>"0"}, "commit"=>"Create Article"}
New article: {"id"=>nil, "title"=>"Debugging Rails", "body"=>"I'm learning how to print in logs.", "published"=>false, "created_at"=>nil, "updated_at"=>nil}
Article should be valid: true
   (0.0ms)  begin transaction
  ↳ app/controllers/articles_controller.rb:31
  Article Create (0.5ms)  INSERT INTO "articles" ("title", "body", "published", "created_at", "updated_at") VALUES (?, ?, ?, ?, ?)  [["title", "Debugging Rails"], ["body", "I'm learning how to print in logs."], ["published", 0], ["created_at", "2018-10-19 00:09:23.216549"], ["updated_at", "2018-10-19 00:09:23.216549"]]
  ↳ app/controllers/articles_controller.rb:31
   (2.3ms)  commit transaction
  ↳ app/controllers/articles_controller.rb:31
The article was saved and now the user is going to be redirected...
Redirected to http://localhost:3000/articles/1
Completed 302 Found in 4ms (ActiveRecord: 0.8ms)

Adding extra logging like this makes it easy to search for unexpected or unusual behavior in your logs. If you add extra logging, be sure to make sensible use of log levels to avoid filling your production logs with useless trivia.

2.4 Verbose Query Logs

When looking at database query output in logs, it may not be immediately clear why multiple database queries are triggered when a single method is called:

irb(main):001:0> Article.pamplemousse
  Article Load (0.4ms)  SELECT "articles".* FROM "articles"
  Comment Load (0.2ms)  SELECT "comments".* FROM "comments" WHERE "comments"."article_id" = ?  [["article_id", 1]]
  Comment Load (0.1ms)  SELECT "comments".* FROM "comments" WHERE "comments"."article_id" = ?  [["article_id", 2]]
  Comment Load (0.1ms)  SELECT "comments".* FROM "comments" WHERE "comments"."article_id" = ?  [["article_id", 3]]
=> #<Comment id: 2, author: "1", body: "Well, actually...", article_id: 1, created_at: "2018-10-19 00:56:10", updated_at: "2018-10-19 00:56:10">

After running ActiveRecord::Base.verbose_query_logs = true in the bin/rails console session to enable verbose query logs and running the method again, it becomes obvious what single line of code is generating all these discrete database calls:

irb(main):003:0> Article.pamplemousse
  Article Load (0.2ms)  SELECT "articles".* FROM "articles"
  ↳ app/models/article.rb:5
  Comment Load (0.1ms)  SELECT "comments".* FROM "comments" WHERE "comments"."article_id" = ?  [["article_id", 1]]
  ↳ app/models/article.rb:6
  Comment Load (0.1ms)  SELECT "comments".* FROM "comments" WHERE "comments"."article_id" = ?  [["article_id", 2]]
  ↳ app/models/article.rb:6
  Comment Load (0.1ms)  SELECT "comments".* FROM "comments" WHERE "comments"."article_id" = ?  [["article_id", 3]]
  ↳ app/models/article.rb:6
=> #<Comment id: 2, author: "1", body: "Well, actually...", article_id: 1, created_at: "2018-10-19 00:56:10", updated_at: "2018-10-19 00:56:10">

Below each database statement you can see arrows pointing to the specific source filename (and line number) of the method that resulted in a database call. This can help you identify and address performance problems caused by N+1 queries: single database queries that generates multiple additional queries.

Verbose query logs are enabled by default in the development environment logs after Rails 5.2.

We recommend against using this setting in production environments. It relies on Ruby's Kernel#caller method which tends to allocate a lot of memory in order to generate stacktraces of method calls.

2.5 Tagged Logging

When running multi-user, multi-account applications, it's often useful to be able to filter the logs using some custom rules. TaggedLogging in Active Support helps you do exactly that by stamping log lines with subdomains, request ids, and anything else to aid debugging such applications.

logger = ActiveSupport::TaggedLogging.new(Logger.new(STDOUT))
logger.tagged("BCX") { logger.info "Stuff" }                            # Logs "[BCX] Stuff"
logger.tagged("BCX", "Jason") { logger.info "Stuff" }                   # Logs "[BCX] [Jason] Stuff"
logger.tagged("BCX") { logger.tagged("Jason") { logger.info "Stuff" } } # Logs "[BCX] [Jason] Stuff"

2.6 Impact of Logs on Performance

Logging will always have a small impact on the performance of your Rails app, particularly when logging to disk. Additionally, there are a few subtleties:

Using the :debug level will have a greater performance penalty than :fatal, as a far greater number of strings are being evaluated and written to the log output (e.g. disk).

Another potential pitfall is too many calls to Logger in your code:

logger.debug "Person attributes hash: #{@person.attributes.inspect}"

In the above example, there will be a performance impact even if the allowed output level doesn't include debug. The reason is that Ruby has to evaluate these strings, which includes instantiating the somewhat heavy String object and interpolating the variables.

Therefore, it's recommended to pass blocks to the logger methods, as these are only evaluated if the output level is the same as — or included in — the allowed level (i.e. lazy loading). The same code rewritten would be:

logger.debug {"Person attributes hash: #{@person.attributes.inspect}"}

The contents of the block, and therefore the string interpolation, are only evaluated if debug is enabled. This performance savings are only really noticeable with large amounts of logging, but it's a good practice to employ.

This section was written by Jon Cairns at a StackOverflow answer and it is licensed under cc by-sa 4.0.

3 Debugging with the debug gem

When your code is behaving in unexpected ways, you can try printing to logs or the console to diagnose the problem. Unfortunately, there are times when this sort of error tracking is not effective in finding the root cause of a problem. When you actually need to journey into your running source code, the debugger is your best companion.

The debugger can also help you if you want to learn about the Rails source code but don't know where to start. Just debug any request to your application and use this guide to learn how to move from the code you have written into the underlying Rails code.

Rails 7 includes the debug gem in the Gemfile of new applications generated by CRuby. By default, it is ready in the development and test environments. Please check its documentation for usage.

3.1 Entering a Debugging Session

By default, a debugging session will start after the debug library is required, which happens when your app boots. But don't worry, the session won't interfere your program.

To enter the debugging session, you can use binding.break and its aliases: binding.b and debugger. The following examples will use debugger:

class PostsController < ApplicationController
  before_action :set_post, only: %i[ show edit update destroy ]

  # GET /posts or /posts.json
  def index
    @posts = Post.all
  # ...

Once your app evaluates the debugging statement, it'll enter the debugging session:

Processing by PostsController#index as HTML
[2, 11] in ~/projects/rails-guide-example/app/controllers/posts_controller.rb
     2|   before_action :set_post, only: %i[ show edit update destroy ]
     4|   # GET /posts or /posts.json
     5|   def index
     6|     @posts = Post.all
=>   7|     debugger
     8|   end
    10|   # GET /posts/1 or /posts/1.json
    11|   def show
=>#0    PostsController#index at ~/projects/rails-guide-example/app/controllers/posts_controller.rb:7
  #1    ActionController::BasicImplicitRender#send_action(method="index", args=[]) at ~/.rbenv/versions/3.0.1/lib/ruby/gems/3.0.0/gems/actionpack-7.0.0.alpha2/lib/action_controller/metal/basic_implicit_render.rb:6
  # and 72 frames (use `bt' command for all frames)

3.2 The Context

After entering the debugging session, you can type in Ruby code as you're in a Rails console or IRB.

(rdbg) @posts    # ruby
(rdbg) self

You can also use p or pp command to evaluate Ruby expressions (e.g. when a variable name conflicts with a debugger command).

(rdbg) p headers    # command
=> {"X-Frame-Options"=>"SAMEORIGIN", "X-XSS-Protection"=>"1; mode=block", "X-Content-Type-Options"=>"nosniff", "X-Download-Options"=>"noopen", "X-Permitted-Cross-Domain-Policies"=>"none", "Referrer-Policy"=>"strict-origin-when-cross-origin"}
(rdbg) pp headers    # command
 "X-XSS-Protection"=>"1; mode=block",

Besides direct evaluation, debugger also helps you collect rich amount of information through different commands. Just to name a few here:

  • info (or i) - Information about current frame.
  • backtrace (or bt) - Backtrace (with additional information).
  • outline (or o, ls) - Available methods, constants, local variables, and instance variables in the current scope.
3.2.1 The info command

It'll give you an overview of the values of local and instance variables that are visible from the current frame.

(rdbg) info    # command
%self = #<PostsController:0x0000000000af78>
@_action_has_layout = true
@_action_name = "index"
@_config = {}
@_lookup_context = #<ActionView::LookupContext:0x00007fd91a037e38 @details_key=nil, @digest_cache=...
@_request = #<ActionDispatch::Request GET "http://localhost:3000/posts" for>
@_response = #<ActionDispatch::Response:0x00007fd91a03ea08 @mon_data=#<Monitor:0x00007fd91a03e8c8>...
@_response_body = nil
@_routes = nil
@marked_for_same_origin_verification = true
@posts = []
@rendered_format = nil
3.2.2 The backtrace command

When used without any options, it lists all the frames on the stack:

=>#0    PostsController#index at ~/projects/rails-guide-example/app/controllers/posts_controller.rb:7
  #1    ActionController::BasicImplicitRender#send_action(method="index", args=[]) at ~/.rbenv/versions/3.0.1/lib/ruby/gems/3.0.0/gems/actionpack-7.0.0.alpha2/lib/action_controller/metal/basic_implicit_render.rb:6
  #2    AbstractController::Base#process_action(method_name="index", args=[]) at ~/.rbenv/versions/3.0.1/lib/ruby/gems/3.0.0/gems/actionpack-7.0.0.alpha2/lib/abstract_controller/base.rb:214
  #3    ActionController::Rendering#process_action(#arg_rest=nil) at ~/.rbenv/versions/3.0.1/lib/ruby/gems/3.0.0/gems/actionpack-7.0.0.alpha2/lib/action_controller/metal/rendering.rb:53
  #4    block in process_action at ~/.rbenv/versions/3.0.1/lib/ruby/gems/3.0.0/gems/actionpack-7.0.0.alpha2/lib/abstract_controller/callbacks.rb:221
  #5    block in run_callbacks at ~/.rbenv/versions/3.0.1/lib/ruby/gems/3.0.0/gems/activesupport-7.0.0.alpha2/lib/active_support/callbacks.rb:118
  #6    ActionText::Rendering::ClassMethods#with_renderer(renderer=#<PostsController:0x0000000000af78>) at ~/.rbenv/versions/3.0.1/lib/ruby/gems/3.0.0/gems/actiontext-7.0.0.alpha2/lib/action_text/rendering.rb:20
  #7    block {|controller=#<PostsController:0x0000000000af78>, action=#<Proc:0x00007fd91985f1c0 /Users/st0012/...|} in <class:Engine> (4 levels) at ~/.rbenv/versions/3.0.1/lib/ruby/gems/3.0.0/gems/actiontext-7.0.0.alpha2/lib/action_text/engine.rb:69
  #8    [C] BasicObject#instance_exec at ~/.rbenv/versions/3.0.1/lib/ruby/gems/3.0.0/gems/activesupport-7.0.0.alpha2/lib/active_support/callbacks.rb:127
  ..... and more

Every frame comes with:

  • Frame identifier
  • Call location
  • Additional information (e.g. block or method arguments)

This will give you a great sense about what's happening in your app. However, you probably will notice that:

  • There are too many frames (usually 50+ in a Rails app).
  • Most of the frames are from Rails or other libraries you use.

Don't worry, the backtrace command provides 2 options to help you filter frames:

  • backtrace [num] - only show num numbers of frames, e.g. backtrace 10 .
  • backtrace /pattern/ - only show frames with identifier or location that matches the pattern, e.g. backtrace /MyModel/.

It's also possible to use these options together: backtrace [num] /pattern/.

3.2.3 The outline command

This command is similar to pry and irb's ls command. It will show you what's accessible from you current scope, including:

  • Local variables
  • Instance variables
  • Class variables
  • Methods & their sources
  • ...etc.
ActiveSupport::Configurable#methods: config
  action_methods  action_name  action_name=  available_action?  controller_path  inspect
  content_type       content_type=  controller_name  dispatch          headers
  location           location=      media_type       middleware_stack  middleware_stack=
  middleware_stack?  performed?     request          request=          reset_session
  response           response=      response_body=   response_code     session
  set_request!       set_response!  status           status=           to_a
  _prefixes  any_templates?  append_view_path   details_for_lookup  formats     formats=  locale
  locale=    lookup_context  prepend_view_path  template_exists?    view_paths
AbstractController::Rendering#methods: view_assigns

# .....

PostsController#methods: create  destroy  edit  index  new  show  update
instance variables:
  @_action_has_layout  @_action_name    @_config  @_lookup_context                      @_request
  @_response           @_response_body  @_routes  @marked_for_same_origin_verification  @posts
class variables: @@raise_on_missing_translations  @@raise_on_open_redirects

You can find more commands and configuration options from its documentation.

3.2.4 Autoloading Caveat

Debugging with debug works fine most of the time, but there's an edge case: If you evaluate an expression in the console that autoloads a namespace defined in a file, constants in that namespace won't be found.

For example, if the application has these two files:

# hotel.rb
class Hotel

# hotel/pricing.rb
module Hotel::Pricing

and Hotel is not yet loaded, then

(rdbg) p Hotel::Pricing

will raise a NameError. In some cases, Ruby will be able to resolve an unintended constant in a different scope.

If you hit this, please restart your debugging session with eager loading enabled (config.eager_load = true).

Stepping commands line next, continue, etc., do not present this issue. Namespaces defined implicitly only by subdirectories are not subject to this issue either.

See ruby/debug#408 for details.

4 Debugging with the web-console gem

Web Console is a bit like debug, but it runs in the browser. In any page you are developing, you can request a console in the context of a view or a controller. The console would be rendered next to your HTML content.

4.1 Console

Inside any controller action or view, you can invoke the console by calling the console method.

For example, in a controller:

class PostsController < ApplicationController
  def new
    @post = Post.new

Or in a view:

<% console %>

<h2>New Post</h2>

This will render a console inside your view. You don't need to care about the location of the console call; it won't be rendered on the spot of its invocation but next to your HTML content.

The console executes pure Ruby code: You can define and instantiate custom classes, create new models, and inspect variables.

Only one console can be rendered per request. Otherwise web-console will raise an error on the second console invocation.

4.2 Inspecting Variables

You can invoke instance_variables to list all the instance variables available in your context. If you want to list all the local variables, you can do that with local_variables.

4.3 Settings

  • config.web_console.allowed_ips: Authorized list of IPv4 or IPv6 addresses and networks (defaults:, ::1).
  • config.web_console.whiny_requests: Log a message when a console rendering is prevented (defaults: true).

Since web-console evaluates plain Ruby code remotely on the server, don't try to use it in production.

5 Debugging Memory Leaks

A Ruby application (on Rails or not), can leak memory — either in the Ruby code or at the C code level.

In this section, you will learn how to find and fix such leaks by using tools such as Valgrind.

5.1 Valgrind

Valgrind is an application for detecting C-based memory leaks and race conditions.

There are Valgrind tools that can automatically detect many memory management and threading bugs, and profile your programs in detail. For example, if a C extension in the interpreter calls malloc() but doesn't properly call free(), this memory won't be available until the app terminates.

For further information on how to install Valgrind and use with Ruby, refer to Valgrind and Ruby by Evan Weaver.

5.2 Find a Memory Leak

There is an excellent article about detecting and fixing memory leaks at Derailed, which you can read here.

6 Plugins for Debugging

There are some Rails plugins to help you to find errors and debug your application. Here is a list of useful plugins for debugging:

  • Query Trace Adds query origin tracing to your logs.
  • Exception Notifier Provides a mailer object and a default set of templates for sending email notifications when errors occur in a Rails application.
  • Better Errors Replaces the standard Rails error page with a new one containing more contextual information, like source code and variable inspection.
  • RailsPanel Chrome extension for Rails development that will end your tailing of development.log. Have all information about your Rails app requests in the browser — in the Developer Tools panel. Provides insight to db/rendering/total times, parameter list, rendered views and more.
  • Pry An IRB alternative and runtime developer console.

7 References


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