asserting scope in a model method

# lets say this is a comments model, acts_as_tree
# used like this @post.comments.find_roots

   def self.find_roots(options = {})
     with_scope :find => options do
       self.find(:conditions => ['parent_id IS NULL'], :order => :position )
     end
   end

# Question: how can i enforce any call is scoped by project_id?
# (I dont want to allow Post.find_roots, it must have a project_id)