tgaff / rails-debugging

pry and other ruby on rails debugging techniques

debugging in ruby and rails

Intro

Objectives
Be able to use pry as a REPL
Be able to enter objects and make changes
Be able to use pry as a debugger
Be able to work with pry-rails and better-errors

pry as REPL

Pry can replace IRB as your ruby REPL of choice.

[1] pry(main)> 1 + 1
=> 2
[2] pry(main)> "hello".public_methods
=> [:<=>,
 :==,
 :===,
 :eql?,
 :hash,
 :casecmp,
 :+,
 :*,
 :%,
 :[],
 :[]=,
 :insert,
 :length,
 :size,
 :bytesize,
 :empty?,
 :=~,
 :match,
 :succ,
 # ... and so on

[3] pry(main)> def say_hello
[3] pry(main)*   puts 'helllllo'
[3] pry(main)* end
=> :say_hello
[4] pry(main)> say_hello
helllllo
=> nil

Oops, that looks like an error. I misspeeled 'hello'.

[6] pry(main)> show-method say_hello

From: (pry) @ line 3:
Owner: Object
Visibility: public
Number of lines: 3

def say_hello
  puts 'helllllo'
end

Yep, that's wrong. I'd better edit that.

[7] pry(main)> edit say_hello

At this point if you're following along one's editor would have popped up and I'd edit the method. You can change the editor pry uses; see exercise 0.

Pry offers a lot more than just easy access to ruby methods though. You can easily enter into objects and view or edit their internal methods using commands much like those you use in your terminal.

[13] pry(main)> cd Hash
[14] pry(Hash):1> ls
Object.methods: yaml_tag
Hash.methods: []  try_convert
Hash#methods:
  ==     compare_by_identity   default_proc=  each_pair   flatten     index    key?     merge!             reject   shift  to_hash  values_at
  []     compare_by_identity?  delete         each_value  has_key?    inspect  keys     pretty_print        reject!  size   to_s
  []=    default               delete_if      empty?      has_value?  invert   length   pretty_print_cycle  replace  store  update
  assoc  default=              each           eql?        hash        keep_if  member?  rassoc              select   to_a   value?
  clear  default_proc          each_key       fetch       include?    key      merge    rehash              select!  to_h   values
locals: _  __  _dir_  _ex_  _file_  _in_  _out_  _pry_

Exercise 0 - 2

• Follow along in exercise 0 to install and configure pry to use your favorite editor.
• Next learn about cd and ls in exercise1.
• Next move on to exercise 2 and fix Cow#say_hello method in pry.

Pry and Rails

That's enough about Pry's awesome REPLabilities.
You're all working on Rails projects these days so let's look at how you can use Pry in Rails.

[14] pry(#<Book>):2> b = Book.new
=> #<Book:0x007fb7d8e79680
 id: nil,
 title: nil,
 year_published: nil,
 author: nil,
 available: nil,
 genre: nil,
 image_url: nil,
 created_at: nil,
 updated_at: nil>
[15] pry(#<Book>):2> b.title = "The Hitchhikers Guide to the Galaxy"
=> "The Hitchhikers Guide to the Galaxy"
[16] pry(#<Book>):2> cd b
[17] pry(#<Book>):3> title
=> "The Hitchhikers Guide to the Galaxy"


[31] pry(BooksController):1> find-method show
ActionController::Rescue
ActionController::Rescue#show_detailed_exceptions?
BooksController
BooksController#show
[32] pry(BooksController):1> edit BooksController

In order to get this working though you'll first need to do a few things.

1. First add pry-rails and pry-byebug to your gemfile
2. bundle install
3. Now instead of using debugger in your ruby code to pause server-execution and debug, type binding.pry

It'll also help if you add a few shortcut commands so you don't have to type continue, step etc.

Debugging with binding.pry

Since pry is a debugger let's talk about a few debugger basics.
Most debuggers have something called a breakpoint which is the point at which execution of the code is paused and you're able to inspect the current state. Some breakpoints are created by inserting a debugger command. In rails that command is debugger, but once we install pry we'll use binding.pry instead.

We can also use the debugger's break command to add more break-points.

break

• halt execution and debug at the specified line/condition

break 10                      # stop at line 10
break BooksController#index   # stop when #index is called
break 10 if x > 3

continue

• continue program execution from where it stopped
• exit the debugger (until another breakpoint is hit)

next

• allow code in the current line to complete
• stop at the next line in the same file or level (if possible)
• You can think of next as staying at the same "level".
• commands on the current line are completed, before you can use the debugger commands again.

step

• step into the code as it executes
• You can think of step as "going-deeper".

 arr.sort.uniq
 puts arr

• when paused on the above line step would show both the code in the sort and uniq methods. On the otherhand, next would just stop at the next puts

pry glossary

A few commands you might make use of include

• edit - edit a file or method
• cd - change current scope
• ls - list variables and methods
• show-method - display a method's code
• find-method - find all methods with this name
• help -
• watch - print a message whenever a variable changes
• show-doc - Show documentation for a specific method

better errors

Better errors is a replacement for the rails error page that adds:

• a clearer stacktrace
• a fully functional REPL console
• clearer listings of params and instance variables
• souce code inspection

annotate

The annotate gem can make working with your models a little easier as it adds some automatic documentation.

Just install the gem and run the annotate command in your terminal.

gem 'annotate'

Schema information will be added as comments to the top of each of your models. You should run annotate after each migration.

# == Schema Information
#
# Table name: books
#
#  id             :integer          not null, primary key
#  title          :string
#  year_published :integer
#  author         :string
#  available      :boolean
#  genre          :string
#  image_url      :string
#  created_at     :datetime         not null
#  updated_at     :datetime         not null
#