How to add subcollection to a document in Firebase Cloud Firestore

Question

The documentation does not have any examples on how to add a subcollection to a document. I know how to add document to a collection and how to add data to a document, but h

Answer 1

Here's a variation where the subcollection is storing ID values at the collection level, rather than within a document where the subcollection is a field there with additional data.

This is useful for connecting a 1-to-Many ID mapping w/out having to drill through an additional document:

function fireAddStudentToClassroom(studentUserId, classroomId) {

    var db = firebase.firestore();
    var studentsClassroomRef =
        db.collection('student_class').doc(classroomId)
          .collection('students');

    studentsClassroomRef
        .doc(studentUserId)
        .set({})
        .then(function () {
            console.log('Document Added ');
        })
        .catch(function (error) {
            console.error('Error adding document: ', error);
        });
}

Thanks to @Alex's answer

This answer a bit off from the original question here, where it explicitly asks for adding a collection to a document. However, after searching for a solution for this scenario and not finding any mention in docs or on SO, this post seems like a reasonable place to share the findings