Firestore subscribe to subcollection that might not yet exist

April 3, 2022

I’ve got a firebase app where users share a calendar. My firestore collection is structured as follows:

calendarEvents/{year}/monthName/{documentId}

How do I subscribe users to a certain subcollection, say

calendarEvents/{2022}/May

that might not even exist yet, as nobody has created an event for May yet.

If not possible (I don’t think it is), how to gracefully achieve such a behavior? The only thing that comes to mind is having separate collection with a single special document that tracks populated months for each year, so basically:

populatedMonths/{theOnlyDocInTheEntireCollection}: {
2021: [1, 2, 5, 12];
2022: [1, 2, 3, 4 ];
}

>Solution :

You can add onSnapshot() listener to collections that do not exist. The querySnapshot returned by listener would just be empty in such cases:

db.collection("col/doc/subcol").onSnapshot((querySnapshot) => {
  console.log(`${querySnapshot.size} documents`)
  // If 0, then collection is either missing or is empty
  // else collection exists and has documents, render them in UI
});