Issue with polymorphic associations

polymorphic.rb

class Task < ActiveRecord::Base
  has_one    :notification_preference, as: :notifee  
end

class NotificationPreference < ActiveRecord::Base
  belongs_to :notifee, polymorphic: true  
end

# Code above keeps on looking for notifee_id on Task.
# The foreign keys are obviously in the NotificationPreference table (notifee_id and notifee_type)
# But I only want Task to have one associated notification preference.

# how do i do this?