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

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:

You'll see something like this:

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

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:

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:

Will be rendered as follows:

Rails has built-in support to debug RJS, to active it, set ActionView::Base.debug_rjs to true, this will specify whether RJS responses should be wrapped in a try/catch block that alert()s the caught exception (and then re-raises it).

To enable it, add the following in the Rails::Initializer do |config| block inside environment.rb:

Or, at any time, setting ActionView::Base.debug_rjs to true:

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

Rails makes use of Ruby's standard logger 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:

Or in the Initializer section, add any of the following

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 ActiveRecord::Base.logger.level method.

The available log levels are: :debug, :info, :warn, :error, and :fatal, corresponding to the log level numbers from 0 up to 4 respectively. To change the default log level, use

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.

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

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

Here's an example of the log generated by this method:

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.

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.

The debugger used by Rails, ruby-debug, comes as a gem. To install it, just run:

In case you want to download a particular version or get the source code, refer to the project's page on rubyforge.

Rails has had built-in support for ruby-debug since Rails 2.0. Inside any Rails application you can invoke the debugger by calling the debugger method.

Here's an example:

If you see the message in the console or logs:

Make sure you have started your web server with the option —debugger:

In order to use Rails debugging you'll need to be running either WEBrick or Mongrel. For the moment, no alternative servers are supported.

As soon as your application calls the debugger 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 ruby-debug's prompt (rdb:n). The n is the thread number. The prompt will also show you the next line of code that is waiting to run.

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:

Now it's time to explore and dig into your application. A good place to start is by asking the debugger for help… so type: help (You didn't see that coming, right?)

The next command to learn is one of the most useful: list. You can also abbreviate ruby-debug commands by supplying just enough letters to distinguish them from other commands, so you can also use l for the list command.

This command shows you where you are in the code by printing 10 lines centered around the current line; the current line in this particular case is line 6 and is marked by ⇒.

If you repeat the list command, this time using just l, the next ten lines of the file will be printed out.

And so on until the end of the current file. When the end of file is reached, the list command will start again from the beginning of the file and continue again up to the end, treating the file as a circular buffer.

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

ruby-debug creates a content when a stopping point or an event is reached. The context has information about the suspended program which enables a 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.

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.

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.

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:

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.

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:

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).

And then ask again for the instance_variables:

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

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

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

You can also inspect for an object method this way:

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.

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).

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 ruby-debug.

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 debugger statement:

With the code stopped, take a look around:

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

@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:

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.

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:

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.

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..

You can also enable or disable breakpoints:

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 is no handler for it.

To list all active catchpoints use catch.

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

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

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.

There are some settings that can be configured in ruby-debug to make it easier to debug your code. Here are a few of the available options:

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

Here's a good start for an .rdebugrc:

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 Bleak House and Valgrind debugging tools.

BleakHouse is a library for finding memory leaks.

If a Ruby object does not go out of scope, the Ruby Garbage Collector won't sweep it since it is referenced somewhere. Leaks like this can grow slowly and your application will consume more and more memory, gradually affecting the overall system performance. This tool will help you find leaks on the Ruby heap.

To install it run:

Then setup you application for profiling. Then add the following at the bottom of config/environment.rb:

Start a server instance with BleakHouse integration:

Make sure to run a couple hundred requests to get better data samples, then press CTRL-C. The server will stop and Bleak House will produce a dumpfile in /tmp:

To analyze it, just run the listed command. The top 20 leakiest lines will be listed:

This way you can find where your application is leaking memory and fix it.

If BleakHouse doesn't report any heap growth but you still have memory growth, you might have a broken C extension, or real leak in the interpreter. In that case, try using Valgrind to investigate further.

Valgrind 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, a C extension in the interpreter calls malloc() but is 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.

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

Lighthouse ticket