def retract(voted_by)
      mark = votes.find_by(voted_by_id: voted_by.id)
      mark && remove_vote_mark(mark)
    end
def retract(voted_by)
      mark = votes.find_by(voted_by_id: voted_by.id)
      mark && remove_vote_mark(mark)
    end
Removes previously added vote.

 @param [Mongoid::Document] voted_by object from which the vote was done
 @return [Boolean] success flag