Google Firestore: Store Document Id as Reference or String

Do you store user_id property for a document as Reference or String?

item {
    name: "Test"
    user_id: "2i5uqlufac0QvPw1YRiE"
    user_ref: "/users/2i5uqlufac0QvPw1YRiE"
}

If you use the property as part of security rules (e.g. only item owner can read the document), you need to use String.

service cloud.firestore {
  match /databases/{database}/documents {
    match /items/{item_id} {
        allow read: if request.auth.uid == resource.data.user_id;
        // the following doesn't work
        // allow read: if request.auth.uid == resource.data.user_ref.id;
        // allow read: if request.auth.uid == get(resource.data.user_ref).id;
    }
  }
}

NOTE: Based on get and Resource, get(resource.data.user_ref).id should work (but it doesn't, a bug maybe?)

Using reference provides context, as in you know the collection path is /users/*, which comes with pros and cons.

• user_ref will return DocumentReference, else you need to construct firestore.collection("users").document(userId).
• I suspect you need to update all user_ref if you ever change collection name or path.
• user_ref can be used to refer to different collections (e.g. /users or /admins or private_users), just in case all users don't necessary resides in /users collection.