gistfile1.js

this.update = function(collection, query, field, update) {
  // Set up this object to pass in to the mongo update method.
  $set = {};

  // Create the $set object for mongo's update method by passing
  // the doc's 'field' to the 'update' argument.
  $set[field] = _.compose(update, _.reduce)(field.split('.')
  , function(memo, accessor) {
    return memo[accessor] // Get field from the doc.
  }, collection.findOne(query));

  // Update the mongo collection.
  collection.update(query, {$set: $set});
};