QuerySnapshot removing data after limit() reached - firebase

Question

I currently have a query snapshot set up to let me recieve live data from my Firestore instance:
let query = db.collection('Tasks')
.where('active', '==', 1)
.where("starts", ">", +new Date())
.where('user', '==', userId)
.orderBy('starts', 'asc').limit(25);
let observer = query.onSnapshot(async snapshot => {
snapshot.docChanges().forEach(change => {
renderTask(change.doc, change.type);
});
}, err => {
console.trace(err);
});
However, when adding a new "Task" and going over the limit value in the query, ie when the query currently has 25 documents being listened to, it invokes the snapshot listener, saying that a document has been removed. Where change.type === 'removed', but in reality, no documents that were being listened to have changed.
I currently use this method to check if a user has actually deleted a "Task", where active == 0 but when I go over the limit for calling documents in the query it also deletes a random "Task".
I've tried adding an if clause to check if the document data contains active = 0, but this doesn't work and the document (even if it has been changed to active = 0, it still contains active = 1 and I assume Firestore doesn't bother with returning an updated document, as it assumes it's not going to be used? This isn't ideal as well as I would also assume that the listnener for the document would be decoupled as well if firestore believes the document has been removed.
How can I differentiate between documents which have actually been deleted, ie active === 0 and documents that are getting removed due to reaching the query limit threshold?
Many thanks in advance.

Answer 1

This is an interesting use case! There is no out-of-the-box solution. Even by using the oldIndex and newIndex properties, you cannot detect if the document was really deleted from the database ("hard delete") or just removed from the query.
One possible solution would be to query the document to see if it still exists, as shown below:
let observer = query.onSnapshot(
(snapshot) => {
snapshot.docChanges().forEach((change) => {
if (change.type === 'removed') {
const docRef = db.collection('Tasks').doc(change.doc.id);
docRef
.get()
.then(function (doc) {
if (doc.exists) {
console.log('STILL EXISTS');
//Do something...
} else {
console.log('HARD DELETED');
//Do something else...
}
})
} else {
//.....
}
});
},
(err) => {
console.trace(err);
}
);
Note that you should not use a async in query.onSnapshot(async snapshot => {...}) since the observer in not an async function.