observational.rb

class Creditor
  observes :message, :invokes => :use_credits, :on => :create
  observes :conversation, :invokes => :use_credits, :on => :create

  def use_credits(user)
    # use credits
  end
end

class Conversation < ActiveRecord::Base
  belongs_to :creator, :class_name => "User"
  belongs_to :receiver, :class_name => "User"
end

class Message < ActiveRecord::Base
  belongs_to :author, :class_name => "User"
end