From the datastore class, you can get a reference to a collection in order to execute a query or insert new documents into it.
// Get reference to a collection var collection = datastore.collection(name);
It is important to note here that it is not required for a collection to pre exist in order to get reference to it. A collection automatically gets created when you insert first document into it. So you will get an error only if you queried a collection which don't contains any document.