trying to figure out this. Im using firestore and trying to store my random generated numbers/characters when im using .doc() while creating up documents instead of naming them.

Now my problem is i wanna make another collection inside that document. A subcollection that is. How do i go about this? Its super easy when i have a hardcoded variable because then i know directly what document to create it in. But how do i work around it with random generated chars/numbers?

Heres my entire thing

        company_id: CompanyID,
        })```

firestore.collection('books').doc().set({
        company_id: CompanyID,
        })        
        .then(() => {
            firestore.collection('books').doc().collection('messages').add({
            text: 'hej',
            createdAt: firebase.firestore.FieldValue.serverTimestamp(),
            myIdNumber,
            displayName,
            })
        })
        .catch((error: string) => {
            console.error("Error while creating: ", error);
        });```