debugging_rails_applications.md

Debugging Rails Applications
============================

This guide introduces techniques for debugging Ruby on Rails applications.

After reading this guide, you will know:

* The purpose of debugging.
* How to track down problems and issues in your application that your tests aren't identifying.
* The different ways of debugging.
* How to analyze the stack trace.

--------------------------------------------------------------------------------

View Helpers for Debugging
--------------------------

One common task is to inspect the contents of a variable. In Rails, you can do this with three methods:

* `debug`
* `to_yaml`
* `inspect`

### `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:

```html+erb
<%= debug @post %>
<p>
  <b>Title:</b>
  <%= @post.title %>
</p>
```

You'll see something like this:

```yaml
--- !ruby/object:Post
attributes:
  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
```

### `to_yaml`

Displaying an instance variable, or any other object or method, in YAML format can be achieved this way:

```html+erb
<%= simple_format @post.to_yaml %>
<p>
  <b>Title:</b>
  <%= @post.title %>
</p>
```

The `to_yaml` method converts the method to YAML format leaving it more readable, and then the `simple_format` helper is used to render each line as in the console. This is how `debug` method does its magic.

As a result of this, you will have something like this in your view:

```yaml
--- !ruby/object:Post
attributes:
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
```

### `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:

```html+erb
<%= [1, 2, 3, 4, 5].inspect %>
<p>
  <b>Title:</b>
  <%= @post.title %>
</p>
```

Will be rendered as follows:

```
[1, 2, 3, 4, 5]

Title: Rails debugging guide
```

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.

### What is the Logger?

Rails makes use of the `ActiveSupport::Logger` class to write log information. You can also substitute another logger such as `Log4r` if you wish.

You can specify an alternative logger in your `environment.rb` or any environment file:

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

Or in the `Initializer` section, add _any_ of the following

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

TIP: 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.

### Log Levels

When something is logged it's printed into the corresponding log if the log level of the message is equal 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

```ruby
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, but you don't want to flood your production log with unnecessary information.

TIP: The default Rails log level is `info` in production mode and `debug` in development and test mode.

### Sending Messages

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

```ruby
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:

```ruby
class PostsController < ApplicationController
  # ...

  def create
    @post = Post.new(params[:post])
    logger.debug "New post: #{@post.attributes.inspect}"
    logger.debug "Post should be valid: #{@post.valid?}"

    if @post.save
      flash[:notice] = 'Post was successfully created.'
      logger.debug "The post was saved and now the user is going to be redirected..."
      redirect_to(@post)
    else
      render action: "new"
    end
  end

  # ...
end
```

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

```
Processing PostsController#create (for 127.0.0.1 at 2008-09-08 11:52:54) [POST]
  Session ID: BAh7BzoMY3NyZl9pZCIlMDY5MWU1M2I1ZDRjODBlMzkyMWI1OTg2NWQyNzViZjYiCmZsYXNoSUM6J0FjdGl
vbkNvbnRyb2xsZXI6OkZsYXNoOjpGbGFzaEhhc2h7AAY6CkB1c2VkewA=--b18cd92fba90eacf8137e5f6b3b06c4d724596a4
  Parameters: {"commit"=>"Create", "post"=>{"title"=>"Debugging Rails",
 "body"=>"I'm learning how to print in logs!!!", "published"=>"0"},
 "authenticity_token"=>"2059c1286e93402e389127b1153204e0d1e275dd", "action"=>"create", "controller"=>"posts"}
New post: {"updated_at"=>nil, "title"=>"Debugging Rails", "body"=>"I'm learning how to print in logs!!!",
 "published"=>false, "created_at"=>nil}
Post should be valid: true
  Post Create (0.000443)   INSERT INTO "posts" ("updated_at", "title", "body", "published",
 "created_at") VALUES('2008-09-08 14:52:54', 'Debugging Rails',
 'I''m learning how to print in logs!!!', 'f', '2008-09-08 14:52:54')
The post was saved and now the user is going to be redirected...
Redirected to #<Post:0x20af760>
Completed in 0.01224 (81 reqs/sec) | DB: 0.00044 (3%) | 302 Found [http://localhost/posts]
```

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.

### 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 in doing exactly that by stamping log lines with subdomains, request ids, and anything else to aid debugging such applications.

```ruby
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"
```

### Impact of Logs on Performance
Logging will always have a small impact on performance of your rails app, 
        particularly when logging to disk.However, 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 that if you have many calls to `Logger` like this
      in your code:

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

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

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

Debugging with the `byebug` 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 deeper into
Rails code.

### Setup

You can use the `byebug` gem to set breakpoints and step through live code in
Rails. To install it, just run:

```bash
$ gem install byebug
```

Inside any Rails application you can then invoke the debugger by calling the
`byebug` method.

Here's an example:

```ruby
class PeopleController < ApplicationController
  def new
    byebug
    @person = Person.new
  end
end
```

### The Shell

As soon as your application calls the `byebug` method, the debugger will be
started in a debugger shell inside the terminal window where you launched your
application server, and you will be placed at the debugger's prompt `(byebug)`.
Before the prompt, the code around the line that is about to be run will be
shown and the current line is marked by '=>'. Like this:

```
[1, 10] in /PathTo/project/app/controllers/posts_controller.rb
   1  class PostsController < ApplicationController
   2    # GET /posts
   3    # GET /posts.json
   4    def index
   5      byebug
=> 6      @posts = Post.all
   7
   8      respond_to do |format|
   9        format.html # index.html.erb
   10        format.json { render json: @posts }
(byebug)
```

If you got there by a browser request, the browser tab containing the request
will be hung until the debugger has finished and the trace has finished
processing the entire request.

For example:

```bash
@posts = Post.all
(byebug:7)
```

Now it's time to explore and dig into your application. A good place to start is
by asking the debugger for help. Type: `help`

```
(byebug) help
byebug 2.6.0

Type 'help <command-name>' for help on a specific command

Available commands:
backtrace  delete   enable  help       list    pry next  restart  source     up
break      disable  eval    info       method  ps        save     step       var
catch      display  exit    interrupt  next    putl      set      thread
condition  down     finish  irb        p       quit      show     trace
continue   edit     frame   kill       pp      reload    skip     undisplay
```

TIP: To view the help menu for any command use `help <command-name>` at the
debugger prompt. For example: _`help list`_. You can abbreviate any debugging
command by supplying just enough letters to distinguish them from other
commands, so you can also use `l` for the `list` command, for example.

To see the previous ten lines you should type `list-` (or `l-`)

```
(byebug) l-
[1, 10] in /PathTo/project/app/controllers/posts_controller.rb
   1  class PostsController < ApplicationController
   2    # GET /posts
   3    # GET /posts.json
   4    def index
   5      byebug
   6      @posts = Post.all
   7
   8      respond_to do |format|
   9        format.html # index.html.erb
   10        format.json { render json: @posts }
```

This way you can move inside the file, being able to see the code above and over
the line you added the `byebug` call.  Finally, to see where you are in the code
again you can type `list=`

```
(byebug) list=
[1, 10] in /PathTo/project/app/controllers/posts_controller.rb
   1  class PostsController < ApplicationController
   2    # GET /posts
   3    # GET /posts.json
   4    def index
   5      byebug
=> 6      @posts = Post.all
   7
   8      respond_to do |format|
   9        format.html # index.html.erb
   10        format.json { render json: @posts }
```

### The Context

When you start debugging your application, you will be placed in different
contexts as you go through the different parts of the stack.

The debugger creates a context when a stopping point or an event is reached. The
context has information about the suspended program which enables the debugger
to inspect the frame stack, evaluate variables from the perspective of the
debugged program, and contains information about the place where the debugged
program is stopped.

At any time you can call the `backtrace` command (or its alias `where`) to print
the backtrace of the application. This can be very helpful to know how you got
where you are. If you ever wondered about how you got somewhere in your code,
then `backtrace` will supply the answer.

```
(byebug) where
    #0 PostsController.index
       at line /PathTo/project/app/controllers/posts_controller.rb:6
    #1 Kernel.send
       at line /PathTo/project/vendor/rails/actionpack/lib/action_controller/base.rb:1175
    #2 ActionController::Base.perform_action_without_filters
       at line /PathTo/project/vendor/rails/actionpack/lib/action_controller/base.rb:1175
    #3 ActionController::Filters::InstanceMethods.call_filters(chain#ActionController::Fil...,...)
       at line /PathTo/project/vendor/rails/actionpack/lib/action_controller/filters.rb:617
...
```

You move anywhere you want in this trace (thus changing the context) by using
the `frame _n_` command, where _n_ is the specified frame number.

```
(byebug) frame 2
#2 ActionController::Base.perform_action_without_filters
       at line /PathTo/project/vendor/rails/actionpack/lib/action_controller/base.rb:1175
```

The available variables are the same as if you were running the code line by
line. After all, that's what debugging is.

Moving up and down the stack frame: You can use `up [n]` (`u` for abbreviated)
and `down [n]` commands in order to change the context _n_ frames up or down the
stack respectively. _n_ defaults to one. Up in this case is towards
higher-numbered stack frames, and down is towards lower-numbered stack frames.

### Threads

The debugger can list, stop, resume and switch between running threads by using
the command `thread` (or the abbreviated `th`). This command has a handful of
options:

* `thread` shows the current thread.
* `thread list` is used to list all threads and their statuses. The plus +
character and the number indicates the current thread of execution.
* `thread stop _n_` stop thread _n_.
* `thread resume _n_` resumes thread _n_.
* `thread switch _n_` switches the current thread context to _n_.

This command is very helpful, among other occasions, when you are debugging
concurrent threads and need to verify that there are no race conditions in your
code.

### Inspecting Variables

Any expression can be evaluated in the current context. To evaluate an
expression, just type it!

This example shows how you can print the instance_variables defined within the
current context:

```
@posts = Post.all
(byebug) instance_variables
["@_response", "@action_name", "@url", "@_session", "@_cookies",
 "@performed_render", "@_flash", "@template", "@_params",
 "@before_filter_chain_aborted", "@request_origin", "@_headers",
 "@performed_redirect", "@_request"]
```

As you may have figured out, all of the variables that you can access from a
controller are displayed. This list is dynamically updated as you execute code.
For example, run the next line using `next` (you'll learn more about this
command later in this guide).

```
(byebug) next
Processing PostsController#index (for 127.0.0.1 at 2008-09-04 19:51:34) [GET]
  Session ID: BAh7BiIKZmxhc2hJQzonQWN0aW9uQ29udHJvbGxlcjo6Rmxhc2g6OkZsYXNoSGFzaHsABjoKQHVzZWR7AA==--b16e91b992453a8cc201694d660147bba8b0fd0e
  Parameters: {"action"=>"index", "controller"=>"posts"}
/PathToProject/posts_controller.rb:8
respond_to do |format|
```

And then ask again for the instance_variables:

```
(byebug) instance_variables.include? "@posts"
true
```

Now `@posts` is included in the instance variables, because the line defining it
was executed.

TIP: You can also step into **irb** mode with the command `irb` (of course!).
This way an irb session will be started within the context you invoked it. But
be warned: this is an experimental feature.

The `var` method is the most convenient way to show variables and their values:

```
var
(byebug) v[ar] const <object>            show constants of object
(byebug) v[ar] g[lobal]                  show global variables
(byebug) v[ar] i[nstance] <object>       show instance variables of object
(byebug) v[ar] l[ocal]                   show local variables
```

This is a great way to inspect the values of the current context variables. For
example:

```
(byebug) var local
  __dbg_verbose_save => false
```

You can also inspect for an object method this way:

```
(byebug) var instance Post.new
@attributes = {"updated_at"=>nil, "body"=>nil, "title"=>nil, "published"=>nil, "created_at"...
@attributes_cache = {}
@new_record = true
```

TIP: The commands `p` (print) and `pp` (pretty print) can be used to evaluate
Ruby expressions and display the value of variables to the console.

You can use also `display` to start watching variables. This is a good way of
tracking the values of a variable while the execution goes on.

```
(byebug) display @recent_comments
1: @recent_comments =
```

The variables inside the displaying list will be printed with their values after
you move in the stack. To stop displaying a variable use `undisplay _n_` where
_n_ is the variable number (1 in the last example).

### Step by Step

Now you should know where you are in the running trace and be able to print the
available variables. But lets continue and move on with the application
execution.

Use `step` (abbreviated `s`) to continue running your program until the next
logical stopping point and return control to the debugger.

TIP: You can also use `step+ n` and `step- n` to move forward or backward `n`
steps respectively.

You may also use `next` which is similar to step, but function or method calls
that appear within the line of code are executed without stopping. As with
`step`, you may use plus sign to move _n_ steps.

The difference between `next` and `step` is that `step` stops at the next line
of code executed, doing just a single step, while `next` moves to the next line
without descending inside methods.

For example, consider this block of code with an included `byebug` statement:

```ruby
class Author < ActiveRecord::Base
  has_one :editorial
  has_many :comments

  def find_recent_comments(limit = 10)
    byebug
    @recent_comments ||= comments.where("created_at > ?", 1.week.ago).limit(limit)
  end
end
```

```
$ rails console
Loading development environment (Rails 4.0.0)
>> author = Author.first
=> #<Author id: 1, first_name: "Bob", last_name: "Smith", created_at: "2008-07-31 12:46:10", updated_at: "2008-07-31 12:46:10">
>> author.find_recent_comments
/PathTo/project/app/models/author.rb:11
)
[2, 9] in /PathTo/project/app/models/author.rb
   2    has_one :editorial
   3    has_many :comments
   4
   5    def find_recent_comments(limit = 10)
   6      byebug
=> 7      @recent_comments ||= comments.where("created_at > ?", 1.week.ago).limit(limit)
   8    end
   9  end
(byebug)
```

You are at the end of the line, but... was this line executed? You can inspect
the instance variables.

```
(byebug) var instance
@attributes = {"updated_at"=>"2008-07-31 12:46:10", "id"=>"1", "first_name"=>"Bob", "las...
@attributes_cache = {}
```

`@recent_comments` hasn't been defined yet, so it's clear that this line hasn't
been executed yet. Use the `next` command to move on in the code:

```
(byebug) next
/PathTo/project/app/models/author.rb:12
@recent_comments
(byebug:1) var instance
@attributes = {"updated_at"=>"2008-07-31 12:46:10", "id"=>"1", "first_name"=>"Bob", "las...
@attributes_cache = {}
@comments = []
@recent_comments = []
```

Now you can see that the `@comments` relationship was loaded and
`@recent_comments` defined because the line was executed.

If you want to go deeper into the stack trace you can move single steps
through your calling methods and into Rails code. This is one of the best ways
to find bugs in your code, or perhaps in Ruby or Rails.

### Breakpoints

A breakpoint makes your application stop whenever a certain point in the program
is reached. The debugger shell is invoked in that line.

You can add breakpoints dynamically with the command `break` (or just `b`).
There are 3 possible ways of adding breakpoints manually:

* `break line`: set breakpoint in the _line_ in the current source file.
* `break file:line [if expression]`: set breakpoint in the _line_ number inside
the _file_. If an _expression_ is given it must evaluated to _true_ to fire up
the debugger.
* `break class(.|\#)method [if expression]`: set breakpoint in _method_ (. and
\# for class and instance method respectively) defined in _class_. The
_expression_ works the same way as with file:line.


```
(byebug:5) break 10
Breakpoint 1 file /PathTo/project/vendor/rails/actionpack/lib/action_controller/filters.rb, line 10
```

Use `info breakpoints _n_` or `info break _n_` to list breakpoints. If you
supply a number, it lists that breakpoint. Otherwise it lists all breakpoints.

```
(byebug:5) info breakpoints
Num Enb What
  1 y   at filters.rb:10
```

To delete breakpoints: use the command `delete _n_` to remove the breakpoint
number _n_. If no number is specified, it deletes all breakpoints that are
currently active.

```
(byebug:5) delete 1
(byebug:5) info breakpoints
No breakpoints.
```

You can also enable or disable breakpoints:

* `enable breakpoints`: allow a list _breakpoints_ or all of them if no list is
specified, to stop your program. This is the default state when you create a
breakpoint.
* `disable breakpoints`: the _breakpoints_ will have no effect on your program.

### Catching Exceptions

The command `catch exception-name` (or just `cat exception-name`) can be used to
intercept an exception of type _exception-name_ when there would otherwise be no
handler for it.

To list all active catchpoints use `catch`.

### Resuming Execution

There are two ways to resume execution of an application that is stopped in the
debugger:

* `continue` [line-specification] \(or `c`): resume program execution, at the
address where your script last stopped; any breakpoints set at that address are
bypassed. The optional argument line-specification allows you to specify a line
number to set a one-time breakpoint which is deleted when that breakpoint is
reached.
* `finish` [frame-number] \(or `fin`): execute until the selected stack frame
returns. If no frame number is given, the application will run until the
currently selected frame returns. The currently selected frame starts out the
most-recent frame or 0 if no frame positioning (e.g up, down or frame) has been
performed. If a frame number is given it will run until the specified frame
returns.

### Editing

Two commands allow you to open code from the debugger into an editor:

* `edit [file:line]`: edit _file_ using the editor specified by the EDITOR
environment variable. A specific _line_ can also be given.

### Quitting

To exit the debugger, use the `quit` command (abbreviated `q`), or its alias
`exit`.

A simple quit tries to terminate all threads in effect. Therefore your server
will be stopped and you will have to start it again.

### Settings

`byebug` has a few available options to tweak its behaviour:

* `set autoreload`: Reload source code when changed (default: true).
* `set autolist`: Execute `list` command on every breakpoint (default: true).
* `set listsize _n_`: Set number of source lines to list by default to _n_
(default: 10)
* `set forcestep`: Make sure the `next` and `step` commands always move to a new
line.

You can see the full list by using `help set`. Use `help set _subcommand_` to
learn about a particular `set` command.

TIP: You can save these settings in an `.byebugrc` file in your home directory.
The debugger reads these global settings when it starts. For example:

```bash
set forcestep
set listsize 25
```

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 tool
such as Valgrind.

### Valgrind

[Valgrind](http://valgrind.org/) is a Linux-only 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](http://blog.evanweaver.com/articles/2008/02/05/valgrind-and-ruby/)
by Evan Weaver.

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:

* [Footnotes](https://github.com/josevalim/rails-footnotes) Every Rails page has
footnotes that give request information and link back to your source via
TextMate.
* [Query Trace](https://github.com/ntalbott/query_trace/tree/master) Adds query
origin tracing to your logs.
* [Query Reviewer](https://github.com/nesquena/query_reviewer) This rails plugin
not only runs "EXPLAIN" before each of your select queries in development, but
provides a small DIV in the rendered output of each page with the summary of
warnings for each query that it analyzed.
* [Exception Notifier](https://github.com/smartinez87/exception_notification/tree/master)
Provides a mailer object and a default set of templates for sending email
notifications when errors occur in a Rails application.
* [Better Errors](https://github.com/charliesome/better_errors) Replaces the
standard Rails error page with a new one containing more contextual information,
like source code and variable inspection.
* [RailsPanel](https://github.com/dejan/rails_panel) 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.

References
----------

* [ruby-debug Homepage](http://bashdb.sourceforge.net/ruby-debug/home-page.html)
* [debugger Homepage](https://github.com/cldwalker/debugger)
* [byebug Homepage](https://github.com/deivid-rodriguez/byebug)
* [Article: Debugging a Rails application with ruby-debug](http://www.sitepoint.com/debug-rails-app-ruby-debug/)
* [Ryan Bates' debugging ruby (revised) screencast](http://railscasts.com/episodes/54-debugging-ruby-revised)
* [Ryan Bates' stack trace screencast](http://railscasts.com/episodes/24-the-stack-trace)
* [Ryan Bates' logger screencast](http://railscasts.com/episodes/56-the-logger)
* [Debugging with ruby-debug](http://bashdb.sourceforge.net/ruby-debug.html)