What is the best way to store a (potentially) huge list in Firestore?

I have a users collection with a bunch of users and their details. I also have a notifications collection, that the users can query on. I expect the amount of notifications to be at least in the thousands, but probably tens of thousands over the years.

I want the users to be able to mark a notification as "seen". How would I go about this?

I have considered following options:

• Add an array notificationsSeen with references to notification documents to each user document. I'm scared of hitting size limits here though, if a user has seen e.g. 50k notifications.
• Add the same but as a sub-collection in users. I'm not sure how to go about this though, since I only really need one property (notification ID). Do I put the notification ID as the sub-collection doc ID and have no fields on the documents? Do I let Firestore generate a random ID and assign the notification ID as a property on the sub-collection?
• Add an array seenBy with references to user documents to each notification document. Although this will allow users to see which notifications other users have seen, and I don't think I want this.

Hope you can help me out, I'm out of ideas and I am not sure how to implement the best idea I have so far (sub-collection in users), which has also been mentioned as a solution here: https://stackoverflow.com/questions/54881751/firestore-storage-size-limit-how-to-store-large-arrays (but without implementation details).

The only scalable way to store an arbitrarily large list of data in Firestore is using documents in a collection. Array type fields do not scale for growing lists of data because the items will eventually exceed the 1MB size limit for a single document, which will obviously cause problems at scale.

It's OK to have a document with no fields. If all you need to do is record that a document exists in order to check its existence later in that collection, that's fine. You can use the notification ID as the document ID if you are absolutely certain that ID conforms to valid IDs in Firestore. Otherwise, you should give it a random ID, and also put the notification ID as a field in the document so that you can query for it later.

You will want to familiarize yourself with the documentation on Firestore limits, which talks about the maximum size of a document, and also the valid characters for a Firestore document ID.

1st option is not possible, as document size limitation defined by firebase, 2nd and 3rd option are possible but 2nd option is a better to implement this feature, and in 2nd option i'll prefer to set notification id as the document id in the subcollection, but setting notification id as property in document is also valid(this option is better suited for case where there are multiple documents with same id, something like posts collection, where user has multiple posts posted).

It’s possible to build a collection for each user for their notifications. You may delete a user-specific document after the notification is read. You may also add some on_snapshot targets to send out notifications after it’s been added to the collection.