Firestore: Query collection using “where” and only retrieve document IDs

问题

I want to query a firestore documents collection and want only retrieve the document ids with angularfire2, to save costs for reading all documents. As far as I understand it, the following code is retrieving all documents too and is causing read ops on all documents, no matter if I use doc.data() or not. Am I right?

db.collection("cities").where("capital", "==", true).onSnapshot(function(querySnapshot) {
  querySnapshot.forEach(function(doc) {
    console.log(doc.id);
  }
});

So in the documentation it is stated:

For queries other than document reads, such as a request for a list of collection IDs, you are billed for one document read.

Is there a way with angularfire2 to query a collection and retrieve only the document IDs?

Or is there an other way than using angularfire2?

来源：https://stackoverflow.com/questions/49880839/firestore-query-collection-using-where-and-only-retrieve-document-ids