Where to define custom error types in Ruby and/or Rails?

For Gems

I have seen many times that you define exceptions in this way:

gem_dir/lib/gem_name/exceptions.rb

and defined as:

module GemName  class AuthenticationError < StandardError; end  class InvalidUsername < AuthenticationError; endend

an example of this would be something like this in httparty

For Ruby on Rails

Put them in your lib/ folder under a file called exceptions.rb, which would look something like this:

module Exceptions  class AuthenticationError < StandardError; end  class InvalidUsername < AuthenticationError; endend

and you would use it like this:

raise Exceptions::InvalidUsername

in rails you can make app/errors directory

# app/errors/foo_error.rbclass FooError < StandardError; end

restart spring/server and it should pick it up

I think in order to have cohesive source files in your project, you should define errors in the class in which may throw them and nowhere else.

Some heirarchy can be helpful - namespaces are good at keeping redundant strings out of type names - but that's more a matter of taste - there's no need to go overboard provided you have at least one custom exception type in your app which you use throughout to differentiate between 'intentional' and 'accidental' exception cases.