custom_error.rb

require 'active_support/core_ext/array/extract_options'

# This makes it easy to tag all errors generated by the app with the
# CustomError module so they can be handled specifically. The NestedException
# module is also included so that it can wrap errors from other parts
# of the system.
module CustomError
  # Public: Builds a new Error class and optionally includes the
  # NestedException module.
  #
  # Usage:
  #   MyError = CustomError.new do
  #     def message
  #       'custom message'
  #     end
  #   end
  #
  #   # This inherits from StandardError instead of RuntimeError
  #   AnotherError = CustomError.new(StandardError, :nested => true)
  #
  def self.new(*args, &block)
    options = args.extract_options!
    base = args.first || RuntimeError
    mixins = [self]
    mixins << NestedException if options.fetch(:nested, false)

    Class.new(base, &block).class_exec { include *mixins }
  end
end


# Adapted from from: http://nestegg.rubyforge.org/
module NestedException

  # Public: Initializes a new nested error, the original error defaults
  # to the global error variable so that it doesn't need to be explicity
  # provided when re-raising an exception.
  def initialize(message = nil, cause = $!)
    @cause = cause
    super(message || cause && cause.message)
  end

  attr_reader :cause

  def set_backtrace(new_backtrace)
    if cause
      last_line = new_backtrace.last
      cause_lines = cause.backtrace
        .take_while { |line| line != last_line }

      new_backtrace << "cause: #{cause.class.name}: #{cause}"
      new_backtrace.concat(cause_lines)
    end

    super(new_backtrace)
  end

end