model.js

    update: function(id, doc, fn){
      var self = this;
      id = (id instanceof ObjectID || id.toHexString) ? id : ObjectID.createFromHexString(id);
      this._collection.update({_id: id}, {$set: doc}, {upsert: false, safe: true}, function(err){
        if (err) return self._connection._error(err);
        if (fn) return self.findById(id, fn, true);
      });
      return this;
    },