Add a document reference to two different collections - google-cloud-firestore

Question

I'm creating a simple job board site.
You have a JobSeeker, a JobListing and a JobApplication
Both the JobSeeker and the JobListing should have a collection of JobApplications.
When a JobSeeker applies for a job, I want to create a JobApplication document, and add it to both the JobSeeker's collection, and the JobListing's collection.
But that should be a reference to a single document. (ie. if you update it in one place, it should update in the other).
How do I achieve this?
I see according to this answer:
Cloud Firestore multiples document with the same reference
I can add a Reference as a datatype in Firestore - but I'm not exactly sure which method to use to add it.
ie. the collection.add method accepts DocumentData, but I can't see how to set that as a reference?
Can you tell me what syntax to use to:
Create the JobApplication document
Add the document reference to a collection.
Retrieve the document reference from either collection.

Answer 1

Here's the way I ended up solving this:
To set the data:
const docData = {
listingId: "someExistingId",
jobSeekerId: "anotherExistingId",
otherData: "whatever other data goes here",
}
const docRef = await db.collection("job-application-collection")
.add(docData);
await db.collection(`job-seeker-collection/${docData.jobSeekerId}/applications`)
.add({ref:docRef});
await db.collection(`job-listing-collection/${docData.listingId}/applications`)
.add({ref:docRef});
That as, what we do is we create one 'real' document, that goes into the job-application-collection and in the JobSeeker and JobListing collections we add a 'pointer document' that just contains a single field ref, containing the document reference.
To retrieve it (in this example, retrieve all of the applications for a given JobSeeker):
const jobSeekerId = "someJobSeekerId";
const colRef = await db.collection(`job-seeker-collection/$jobSeekerId}/applications`);
const colSnapshot = await colRef.get();
/**
* The docs on the collection are actually just documents containing a reference to the actual JobApplication document.
*/
const docsProms = colSnapshot.docs.map((async (colDocData) => {
const snapshot = await colDocData.data().ref.get();
return {
...snapshot.data(),
id: snapshot.id,
}
}));
const data = await Promise.all(docsProms);
return data;
Pretty straight forward, we get the collection on the JobSeeker document, and then on each of those documents, there is a ref field, which we can use the .get() method to return a document snapshot.