I have many Users that Vote on Requirements

Each user can vote on a requirement on more than one criteria.

So I want to get a list of all of the votes in a criteria, in a two-d array like such. There are some places where there is no vote. In that case the value in the array is an empty string

        User 1      User 2    User 3
Req 1     9           4         5
Req 2     1           ""        ""
Req 3     ""          4         0


I currently get the array using this method but it is very slow as there are thousands of votes.


class Criteria < ActiveRecord::Base
  set_table_name "criteria"
  set_primary_key :criteria_id
  belongs_to :instance
  has_many :votes

  def build_vote_array
    vote_array = Array.new(instance.requirements.count){|req_i|
      Array.new(instance.project.stakeholders.count) {|stake_i|
        vote = self.votes.select{|v|
          v.requirement_id == instance.requirements[req_i].id &&
          v.user_id == instance.project.stakeholders[stake_i].id
        }[0]
        vote == nil ? "" : vote.value
      }
    }

  end
end