ruby on rails - Calculating karma from upvotes/downvotes on a user's posts -

i have user , post model , using acts_as_votable gem (which gives me 'votes' table) upvote/downvote post. want assign karma each user, karma number of upvotes user has gained his/her posts minus total number of downvotes.

currently have instance method on user's model calculate karma:

  def karma     count = 0     self.posts.each |post|       count += post.upvotes.size - post.downvotes.size     end     return count   end 

i don't think above efficient run time of o(2n), because every post 2 additional database queries required, 1 upvotes, , 1 downvotes.

any ideas on how combined above single query, or otherwise make more efficient?

not efficiency, such design not in terms of oop. user model considers beyond scope. karma belongs user , should not tied in else.

a better approach separate "karma" post.

# add "karma" column $ rails g migration addkarmatouser  # or use dedicated table class user < activerecord::base   has_one :karma 

then, use controller or service object change karma. i'll cover controller here simplicity. not use model callbacks cross models.

class postscontroller < applicationcontroller   def upvote     post = post.find(params[:id])     post.upvote # pseudo api of acts_as_votable     current_user.increase_karma   end    def downvote     # ...     current_user.decrease_karma   end end  # model class user < activerecord::base   attr_accessible :karma    def increase_karma(count=1)     update_attribute(:karma, karma + count)   end    def descrease_karma(count=1)     # ...   end end  # view user.karma