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

Question

Is there a best practice for defining custom error types in a Ruby library (gem) or Ruby on Rails application? Specifically:

1. Where do they belong structurally

Answer 1

This is an old question, but I wanted to share how I'm handling custom errors in Rails, including attaching error messages, testing, and how to handle this with ActiveRecord models.

Creating Custom Error

class MyClass
  # create a custome error
  class MissingRequirement < StandardError; end

  def my_instance_method
    raise MyClass::MissingRequirement, "My error msg" unless true   
  end
end

Testing (minitest)

test "should raise MissingRequirement if ____ is missing"
  # should raise an error
  error = assert_raises(MyClass::MissingRequirement) {
    MyClass.new.my_instance_method
  }

  assert error.message = "My error msg"
end

With ActiveRecord

I think it's worth noting that if working with an ActiveRecord model, a popular pattern is to add an error to the model as described below, so that your validations will fail:

def MyModel < ActiveRecord::Base
  validate :code_does_not_contain_hyphens

  def code_does_not_contain_hyphens
    errors.add(:code, "cannot contain hyphens") if code.include?("-")
  end
end

When validations are run, this method will piggy-back onto ActiveRecord's ActiveRecord::RecordInvalid error class and will cause validations to fail.

Hope this helps!