Firestore: What's the pattern for adding new data in Web v9?

If you are trying to get:

• a CollectionReference, then use collection():

const myCol = collection(db, "collection", "doc1", "sub-col1")

• a DocumentReference, then use doc():

const myDoc = doc(db, "collection", "doc1", "sub-col1", "sub-doc1")

The concept remains same. The path to a document has even number of segments e.g. col/doc/sub-col/sub-doc while path to a collection has odd e.g. col/doc/sub-col.

Both the methods will throw an error if invalid number of parameters are passed.

In the name-spaced version (v8), it used to look like:

// DocumentReference
firebase.firestore().doc("col/doc/sub-col/sub-doc")

// CollectionReference
firebase.firestore().collection("col/doc/sub-col")

In essence, you keep adding path segments to the same doc() or collection() methods.

doc(firestore: Firestore, path: string, ...pathSegments: string[]):
// You can also use spread operator with an array

An example with spread operator:

const myDocPath = "users/user1/posts/post1/comments/comment1"
const docRef = doc(db, ...myDocPath.split("/"))

Just make sure you don't have any leading or trailing slash if using spread operator with split().