Custom Error Messages in Ruby

group.rb

class Group
  module Error
    class Standard < StandardError; end

    class AlreadyAMember < Standard
      def message
        "You are already a member of this group."
      end
    end

    class NotPermittedToJoin < Standard
      def message
        "You don't have the proper permissions to join this group."
      end
    end
  end

  def join user
    raise Error::NotPermittedToJoin unless self.permitted?(user)
    raise Error::AlreadyAMember if self.member?(user)
    self.members.create :user => user
  end
end

groups_controller.rb

class GroupsController < ApplicationController
  def join
    @group = Group.find(params[:id])
    @group.join current_user
    flash[:notice] = "Welcome to the Group!"
    redirect_to @group
  rescue Group::Error::Standard => exception
    flash[:error] = exception.message
    render :action => 'request'
  end
end

An improved and more OOP approach to this example can be found here.

I liked this approach... +1

+1
How do I pass params to my custom exception?

Ex:

raise Error::AlreadyAMember.new(user)

And then in the Error class

def initialize(response)
    @response = response
 end