NoSQL structure for conditional push notifications

Question

I'm using Firestore as my NoSQL database and have struggled to try to work out the best structure to allow querying to conditionally send batched push notifications.

Previously I had this type of NoSQL structure (collection/doc/subcollection/doc):

users/ <-- root collection
  [userId]
    { push token }
    items/ <-- sub collection
      [itemId]
        { item data }

I would store the user push token at the users/[userId] level but the actual data for the user in the sub-collection.

The issue I found was querying across all users depending on the data in their sub-collection.

My next solution was to move the sub-collections to their own root collection, and each document has a reference to the users/[userId] that the data belonged to.

users/
  [userId]
    { push token }
items/
  [itemId]
    { item data, [userId] }

This would allow me to run the following logic in a cloud function:

1. Store push tokens in a users root collection
2. Fetch all user docs that have a stored push token
3. Loop over the above user ids, and conditionally fetch item data that share a user id

Is this a good structure for working out which users you need to send a push notification to?

Answer 1

In the NoSQL world, we are structuring a database schema according to the queries that we want to perform. If you're asking if it's better to store the documents in a top-level collection vs. a sub-collection, then please check my answer from the following post:

• What are the benefits of using a root collection in Firestore vs. a subcollection?

So none of those two solutions is better than the other in terms of speed of costs. Regarding speed, Firestore is as fast at level 1 as it is at level 100. Regarding costs, everything in Firestore is about the number of reads you perform. So it doesn't really matter if your query returns N documents from a collection or the same N documents from a sub-collection, the cost will always be the same.

So if one of the schemas you mentioned in your question satisfies your needs, then you can go ahead with it.