user.rb

has_many :user_notifications

NOTIFICATIONS_CONDITION = '(actor_type = \'User\' AND ((secondary_subject_type = \'Video\' AND secondary_subject_id IN (SELECT id FROM videos WHERE user_id = #{id})) OR (secondary_subject_type = \'User\' AND secondary_subject_id = #{id})))' 
 
has_many :notifications, :through => :user_notifications,
                          :finder_sql => 'SELECT timeline_events.* FROM timeline_events
                                           WHERE ' + NOTIFICATIONS_CONDITION + '
                                           ORDER BY timeline_events.created_at DESC'

user_notification.rb

belongs_to :user
belongs_to :notification, :classname => 'TimelineEvent'