Fetch only the document ids for documents that match a query in Firebase Cloud Functions

Question

I know I can retrieve the documents themselves with a query such as:

    const querySnapshot = await db.collection("records")
      .where("color", "==", 'blue')
      .get();

However this would presumably get expensive, and all I really need is the document Ids so that I can write to the documents. I don't really need to read their data.

So, how can I fetch only the document ids for documents that match my query?

Thanks