Advantages of firestore sub-collections

I’ ve got some key points about subcollections that you need to be aware of when modeling your database.

1 – Subcollections give you a more structured database.

2 - Queries are indexed by default: Query performance is proportional to the size of your result set, not your data set. So does not matter the size of your collection, the performance depends on the size of your result set.

3 – Each document has a max size of 1MB. For instance, if you have an array of orders in your customer document, it might be a good idea to create a subcollection of orders to each customer because you cannot foresee how many orders a customer will have. By doing this you don’t need to worry about the max size of your document.

4 – Pricing: Firestore charges you for document reads, writes and deletes. Therefore, when you create many subcollections instead of using arrays in the documents, you will need to perform more read, writes and deletes, thus increasing your bill.

I was wondering about the same thing. The documentation mainly talks about arrays vs sub-collections. My conclusion is that there are no clear advantages of using a sub-collection over a top-level collection. Sub collections had some clear technical limitations before, but I think those are removed with the recent introduction of collection group queries.

Here are some advantages of both approaches:

Sub collection:

• Your database "feels" more structured as you will have less top-level collections listed.
• No need to store a reference/foreign key/id of the parent document, as it is implied by the database structure. You can get to the parent via the sub collection document ref.

Top-level collection:

• Documents are easier to delete. Using sub collections you need to make sure to first delete all sub collection documents before you delete the parent document. There is no API for this so you might need to roll your own helper functions.
• Having the parent id directly in each (sub) document might make it easier to process query results, depending on the application.

To answer the original question about efficiency:

Querying all people with the family 'Smith' from the people top-level collections really is not any slower than asking for all the people in the 'Smith' family sub-collection.

This is explained in the How to Structure Your Data episode of the Get to Know Cloud Firestore video series.

There are some trade-offs between top-level collections and sub-collections to be aware of. Depending on the specific queries you intend to use you may need to create composite indexes to query top-level collections or collection group indexes to query sub-collections. Both these index types count towards the 200 index exemptions limit.

These trade-offs are discussed in detail near the bottom of the Understanding Collection Group Queries blog post and in Maps, Arrays and Subcollections, Oh My! episode of the Get to Know Cloud Firestore video series.

I've linked to the relevant parts of both videos.