def self.helpful_reporters(opts={})
  opts = {:limit => 5}.merge(opts)
  map = <<EOF
    function() {
      emit(this.username, this.votes);
    };
EOF
  reduce = <<EOF
    function(key, values) {
      var total = 0;
      for(var i=0; i<values.length; i++) {
        total += values[i];
      }
      return total;
    };
EOF
  collection.mapreduce(map, reduce, {:query => {:created_at => {"$gte" => 1.week.ago.utc}}, :out => "helpful_reporters"}).find.sort(["value", Mongo::DESCENDING]).limit(opts[:limit]).map do |reporter|
      {:username => reporter["_id"], :occurrences => reporter['value'].to_i}
  end
end