belongs_to  :user
  belongs_to  :assignee, :class_name => "User", :foreign_key => :assigned_to
  belongs_to  :completor, :class_name => "User", :foreign_key => :completed_by
  belongs_to  :asset, :polymorphic => true
  has_many    :activities, :as => :subject, :order => 'created_at DESC'