Trouble batch setting a document field by docId in Firestore

I have been using firebase (firestore) for a while but I’m a little stuck and was wondering if anyone can think of a solution.

On the firestore DB I have a single collection of users, each user has an email address and several other fields. In this instance I am checking if a user email exists and if it does, I want to create a list field for that particular user with a listUid. I am referencing the users by email, grabbing the docId for those users and then trying to set a list field for each of them.

I am not getting any error’s from firestore, it’s simply not updating in the DB for some reason and I can’t figure out where I am going wrong. Thanks in advance

export const addListUidToExistingUserList = (
  { firestore },
  emailArray,
  listUid
) => {
  return async () => {
    let docIds = [];

    emailArray.forEach((emailAddress) => {
      //find users by email (works)
      const query = db
        .collection("users")
        .where("email", "==", emailAddress);

      //get docId's for user with matching email (works)
      query.get().then((querySnapshot) => {
        querySnapshot.forEach((doc) => {
          docIds.push(doc.id);
        });
      });

      //add a new list with corresponding listUid (does not work)
      docIds.forEach((id) => {
        let userRef = db.collection("users").doc(id);
        batch.set(userRef, { lists: [{ listUid }] });
      });
    });
    return await batch.commit();
  };
};

Answer

You are running into this issue because your docIds array is always empty at the time you call docIds.forEach.

That’s because query.get().then runs asynchronously, and so docIds.forEach is not waiting for it to complete.

You could either:

• await query.get().then; or
• Add the docIds.forEach function INSIDE the then callback of query.get.

Here are your possible fixes:

1. await query.get().then

//get docId's for user with matching email (works)
await query.get().then((querySnapshot) => {
  querySnapshot.forEach((doc) => {
    docIds.push(doc.id);
  });
});

OR:

2. docIds.forEach inside then

//get docId's for user with matching email (works)
query.get().then((querySnapshot) => {
  querySnapshot.forEach((doc) => {
    docIds.push(doc.id);
  });

  docIds.forEach((id) => {
    let userRef = db.collection("users").doc(id);
    batch.set(userRef, { lists: [{ listUid }] });
  });
});

Note: Of course, you could also add batch.set directly into your first iteration of querySnapshot.docs.forEach to prevent an unnecessary iteration.