Firebase如何一次性检索文档集合以及每个文档的子集合？

const usersRef = firebase.firestore().collection("users");

usersRef.get().then((querySnapshot) => {
  querySnapshot.forEach((doc) => {
    console.log(doc.id, " => ", doc.data());
  });
});

usersRef.where("age", ">=", 18).get().then((querySnapshot) => {
  querySnapshot.forEach((doc) => {
    console.log(doc.id, " => ", doc.data());
  });
});

usersRef.get().then((querySnapshot) => {
  querySnapshot.forEach((doc) => {
    const subcollectionRef = doc.ref.collection("subcollection");
    subcollectionRef.get().then((subcollectionSnapshot) => {
      subcollectionSnapshot.forEach((subdoc) => {
        console.log(subdoc.id, " => ", subdoc.data());
      });
    });
  });
});