gistfile1.rb

# UPDATED - IT WORKS
# Thanks to "kuadrosx"!
# #mongoid on chat.freenode.net

class Conversation
  include Mongoid::Document
  field :name
  
  embeds_many :messages, as: :messagable, :class_name => 'ConversationMessage'
end

class ConversationMessage
  include Mongoid::Document
  belongs_to  :user
  embedded_in :messagable
end

class ConversationTextMessage < ConversationMessage
  field :body
end

class ConversationInviteTextMessage < ConversationMessage
  belongs_to  :guest, class_name: "User"
end







c = Conversation.first
c.messages << ConversationTextMessage.new(user: User.first, body: "Yo.")
c.messages << ConversationInviteTextMessage.new(user: User.first, guest: User.last)
c.save

c.messages.each do |message|
  puts message.class
end

# => ConversationTextMessage
# => ConversationInviteTextMessage