ruby on rails - single table inheritance with embeds_one mogoid -

i have model

class post   include mongoid::document   include mongoid::timestamps    embeds_one :comment end 

and have comment class

class comment   include mongoid::document   include mongoid::timestamps    embedded_in :post    field :title   field :description end 

and have class inherited comment

class recentcomment < comment   # methods end 

now want able create recentcomment through post if post.last.build_comment(:_type => "recentcomment") new comment not of _type:"recentcomment", , if post.last.build_recent_comment, gives me error saying sth undefined method build_recent_comment post class. if post had references_many :comments should have done post.last.build_comments({}, recentcomment) without problems. don't know how build object recentcomment class in case. if that'd gr8!

note: using gem 'mongoid', '~> 2.0.1'

maybe try

class post   include mongoid::document   include mongoid::timestamps    embeds_one :recent_comment, :class_name => comment 

and make comment class polymorphic

class comment   include mongoid::document   include mongoid::timestamps    field :type   validates_inclusion_of :type, :in => ["recent", "other"]