Firestore - Working with Field Masks

Sets in Vocabustudy are split over two collections in Firestore - the actual set content and the metadata of the set. However, there are a couple fields that are duplicated across both collections.

collection	field	description
both	uid	user ID
both	name	set name
both	visibility	visibility of the set
meta_set	creator	the creator's display name
meta_set	collections	basically tags
meta_set	likes	the like count (updated by a script)
meta_set	numTerms	the number of terms (without this field I would have to fetch terms to get the length)
meta_set	nameWords	a list of words in the set name (for "searching")
set	description	an extended description
set	terms	the actual vocabulary in the set

Now, why did I do this? Why can't I have all of the fields in one collection? When users are searching for sets or viewing the sets, all of the information for each document is loaded. the terms field can be significantly large for certain sets, and loading that for each document returned would take significantly longer than simply loading the metadata needed to show a simple link to a set.

This is costly to maintain, as it requires two writes to create, edit, or delete a set and, of course, duplicate data has to be written.

However, the Firestore REST API has once again proved to be very useful!

The documentation for the documents.get method (and a lot of the other methods!) describes a mask query parameter.

That means I can only retrieve certain fields -- thereby decreasing the data egress -- while still storing everything in one document!

For instance, if I wanted to only get the metadata for a set, I could do this:

const setRes = await fetch("https://firestore.googleapis.com/v1/projects/PROJECT_ID/databases/(default)/documents/sets/SET_ID?mask=uid,name,creator,collections,likes,numTerms");
const { fields: setFields } = await setRes.json();

Why isn't this in in the Web SDK? The folks at Firebase actually do have a good reason for this - the Mobile and Web SDKs are designed to be mobile first. If only part of a document was retrieved, the user would face only having that partial document when offline. Even so, that doesn't stop use from using the REST API!

Hopefully this post will help someone out there...