db.eval() and server-side updates on MongoDB with the Ruby driver

controller.rb

  # Records the vote against 2 items.
  # Both parameters are strings
  def record_votes(winner_id, loser_id)
    # grab the connection from the underlying driver
    db = Mongoid::Config::master

    # a future improvement would be to store this away somewhere, perhaps as a constant
    # note: javascript comments added for the post: they are not in the real code
    f = """
function(winner_id, loser_id) {
  // will update one of the documents based on if it won the vote or not
  var update = function(id, win) {
    var doc = db.items.findOne({_id: ObjectId(id)});
    if (!doc) return; // just ignore if can't find it

    // updated automatically be Mongoid but we need to do it ourselves here
    doc.updated_at = new Date();

    // the simple updates that could have been done in-place
    doc.votes++;
    if (win) doc.wins++; else doc.losses++;

    // stats calculations. the actual calculation is not important.
    doc.percent_wins = Math.round(doc.wins / doc.votes * 100);
    doc.percent_losses = Math.round(doc.losses / doc.votes * 100);
    doc.rank = doc.percent_wins * (doc.votes >= #{ItemsController::VOTE_THRESHOLD} ? 100 : 10);

    db.items.save(doc);
  }

  // perform separate updates on each document
  update(winner_id, true);
  update(loser_id, false);
}
    """

    # note $eval has to be first
    db.command({:$eval => f, args: [winner_id, loser_id], nolock: true})
  end

runCommand.txt

> db.runCommand({ $eval: function(abool) { return abool; }, args: [false], nolock: true });
Tue Nov 22 11:55:02 [conn1] query test.$cmd ntoreturn:1 command: { $eval: function (abool) {
    return abool;
}, args: [ false ], nolock: true } reslen:62 183ms
{ "retval" : false, "ok" : 1 }