code: function getCollection(cb) {
      if (collection) {
        setImmediate(function () { cb(null, collection); });
        return;
      }
      MongoClient.connect(url, function (err, db) {
        if (err) {
          return cb(err);
        }
        collection = db.collection(collectionName);
        cb(null, collection);
      });
    }
docs: [END translate]