Ruby custom error classes: inheritance of the message attribute

I can't seem to find much information about custom exception classes.

What I do know

You can declare your custom error class and let it inherit from StandardError, so it can be rescued:

class MyCustomError < StandardError
end

This allows you to raise it using:

raise MyCustomError, "A message"

and later, get that message when rescuing

rescue MyCustomError => e
  puts e.message # => "A message"

What I don't know

I want to give my exception some custom fields, but I want to inherit the message attribute from the parent class. I found out reading on this topic that @message is not an instance variable of the exception class, so I'm worried that my inheritance won't work.

Can anyone give me more details to this? How would I implement a custom error class with an object attribute? Is the following correct:

class MyCustomError < StandardError
  attr_reader :object
  def initialize(message, object)
    super(message)
    @object = object
  end
end

And then:

raise MyCustomError.new(anObject), "A message"

to get:

rescue MyCustomError => e
  puts e.message # => "A message"
  puts e.object # => anObject

will it work, and if it does, is this the correct way of doing things?

raise already sets the message so you don't have to pass it to the constructor:

class MyCustomError < StandardError
  attr_reader :object

  def initialize(object)
    @object = object
  end
end

begin
  raise MyCustomError.new("an object"), "a message"
rescue MyCustomError => e
  puts e.message # => "a message"
  puts e.object # => "an object"
end

I've replaced rescue Exception with rescue MyCustomError, see https://stackoverflow.com/questions/10048173/why-is-it-bad-style-to-rescue-exception-e-in-ruby.

Given what the ruby core documentation of Exception, from which all other errors inherit, states about #message

> Returns the result of invoking exception.to_s. Normally this returns > the exception’s message or name. By supplying a to_str method, > exceptions are agreeing to be used where Strings are expected.

http://ruby-doc.org/core-1.9.3/Exception.html#method-i-message

I would opt for redefining to_s/to_str or the initializer. Here is an example where we want to know, in a mostly human readable way, when an external service has failed to do something.

NOTE: The second strategy below uses the rails pretty string methods, such as demodualize, which may be a little complicated and therefore potentially unwise to do in an exception. You could also add more arguments to the method signature, should you need.

Overriding #to_s Strategy not #to_str, it works differently

module ExternalService
   
  class FailedCRUDError < ::StandardError
    def to_s
      'failed to crud with external service'
    end
  end

  class FailedToCreateError < FailedCRUDError; end
  class FailedToReadError < FailedCRUDError; end
  class FailedToUpdateError < FailedCRUDError; end
  class FailedToDeleteError < FailedCRUDError; end
end

Console Output

begin; raise ExternalService::FailedToCreateError; rescue => e; e.message; end
# => "failed to crud with external service"

begin; raise ExternalService::FailedToCreateError, 'custom message'; rescue => e; e.message; end
# => "failed to crud with external service"

begin; raise ExternalService::FailedToCreateError.new('custom message'); rescue => e; e.message; end
# => "failed to crud with external service"

raise ExternalService::FailedToCreateError
# ExternalService::FailedToCreateError: failed to crud with external service

Overriding #initialize Strategy

This is the strategy closest to implementations I've used in rails. As noted above, it uses the demodualize, underscore, and humanize ActiveSupport methods. But this could be easily removed, as in the previous strategy.

module ExternalService
  class FailedCRUDError < ::StandardError
    def initialize(service_model=nil)
      super("#{self.class.name.demodulize.underscore.humanize} using #{service_model.class}")
    end
  end

  class FailedToCreateError < FailedCRUDError; end
  class FailedToReadError < FailedCRUDError; end
  class FailedToUpdateError < FailedCRUDError; end
  class FailedToDeleteError < FailedCRUDError; end
end

Console Output

begin; raise ExternalService::FailedToCreateError; rescue => e; e.message; end
# => "Failed to create error using NilClass"

begin; raise ExternalService::FailedToCreateError, Object.new; rescue => e; e.message; end
# => "Failed to create error using Object"

begin; raise ExternalService::FailedToCreateError.new(Object.new); rescue => e; e.message; end
# => "Failed to create error using Object"

raise ExternalService::FailedCRUDError
# ExternalService::FailedCRUDError: Failed crud error using NilClass

raise ExternalService::FailedCRUDError.new(Object.new)
# RuntimeError: ExternalService::FailedCRUDError using Object

Demo Tool

This is a demo to show rescuing and messaging of the above implementation. The class raising the exceptions is a fake API to Cloudinary. Just dump one of the above strategies into your rails console, followed by this.

require 'rails' # only needed for second strategy 

module ExternalService
  class FailedCRUDError < ::StandardError
    def initialize(service_model=nil)
      @service_model = service_model
      super("#{self.class.name.demodulize.underscore.humanize} using #{@service_model.class}")
    end
  end

  class FailedToCreateError < FailedCRUDError; end
  class FailedToReadError < FailedCRUDError; end
  class FailedToUpdateError < FailedCRUDError; end
  class FailedToDeleteError < FailedCRUDError; end
end

# Stub service representing 3rd party cloud storage
class Cloudinary

  def initialize(*error_args)
    @error_args = error_args.flatten
  end

  def create_read_update_or_delete
    begin
      try_and_fail
    rescue ExternalService::FailedCRUDError => e
      e.message
    end
  end

  private def try_and_fail
    raise *@error_args
  end
end

errors_map = [
  # Without an arg
  ExternalService::FailedCRUDError,
  ExternalService::FailedToCreateError,
  ExternalService::FailedToReadError,
  ExternalService::FailedToUpdateError,
  ExternalService::FailedToDeleteError,
  # Instantiated without an arg
  ExternalService::FailedCRUDError.new,
  ExternalService::FailedToCreateError.new,
  ExternalService::FailedToReadError.new,
  ExternalService::FailedToUpdateError.new,
  ExternalService::FailedToDeleteError.new,
  # With an arg
  [ExternalService::FailedCRUDError, Object.new],
  [ExternalService::FailedToCreateError, Object.new],
  [ExternalService::FailedToReadError, Object.new],
  [ExternalService::FailedToUpdateError, Object.new],
  [ExternalService::FailedToDeleteError, Object.new],
  # Instantiated with an arg
  ExternalService::FailedCRUDError.new(Object.new),
  ExternalService::FailedToCreateError.new(Object.new),
  ExternalService::FailedToReadError.new(Object.new),
  ExternalService::FailedToUpdateError.new(Object.new),
  ExternalService::FailedToDeleteError.new(Object.new),
].inject({}) do |errors, args|
  begin 
    errors.merge!( args => Cloudinary.new(args).create_read_update_or_delete)
  rescue => e
    binding.pry
  end
end

if defined?(pp) || require('pp')
  pp errors_map
else
  errors_map.each{ |set| puts set.inspect }
end

Your idea is right, but the way you call it is wrong. It should be

raise MyCustomError.new(an_object, "A message")

I wanted to do something similar. I wanted to pass an object to #new and have the message set based on some processing of the passed object. The following works.

class FooError < StandardError
  attr_accessor :message # this is critical!
  def initialize(stuff)
    @message = stuff.reverse
  end
end

begin
  raise FooError.new("!dlroW olleH")
rescue FooError => e
  puts e.message #=> Hello World!
end

Note that if you don't declare attr_accessor :message then it will not work. Addressing the OP's issue, you could also pass the message as an additional argument and store anything you like. The crucial part appears to be overriding #message.