Firebase link data to user

Question

How do I link the data to the user?

In my case I have a comments section on my blog, where the user can post a comment.

The structure looks something like this:

- posts
  - My First Post
    - content: "a big string of the post content"
    - data: "Date Created"
    - image: "Image URL"
    - imagecaption: "Image Caption"
    - comments
      - ???

Now under the comments it would be nice to have something like this:

- comments
  - HbsfJSFJJSF (Comment ID)
    - user: (User Reference)
    - comment: "Nice Blog!"

Now I understand I could so something like this:

- comments
  - HbsfJSFJJSF (Comment ID)
    - user: user_uid
    - comment: "Nice Blog!"

But that has the problem(?) that if the account is deleted (I have that feature), the comment won't be deleted.

Is there a proper way to link the data (comment) to the user such that when the users account is deleted the comment gets deleted, or at least is there a way to delete the comments corresponding to the user when his/her account is deleted?

Answer 1

There is no built-in support in the Firebase Realtime Database for a managed link. So it's up to code that you write.

Frequently this means that you'll have a central function (possibly in Cloud Functions for Firebase) that handles the deletion of a user. This function then calls Firebase Authentication to delete the user, and it updates the database to remove the references to the user.

There is also an open-source project that aims to make this sort of clean-up operation simpler/more reliable: https://github.com/firebase/user-data-protection

Answer 2

Your idea of using user: user_uid under each comment node would work and is known as denormalisation and fanout .

Using this method, you could cascade deletes by performing a query to obtain all comments where the user value is equal to the current user's ID, and delete each one, something like:

var commentsRef = firebase.database().ref('comments');
var userId = firebase.auth().currentUser.uid;

commentsRef.orderByChild('user').equalTo(userId).once('value', function(snapshot) {
  snapshot.forEach(function(childSnapshot) {
    var commentKey = childSnapshot.key;
    commentsRef.child(commentKey).remove();
  });
});

To ensure this is performed behind the scenes when a user is deleted, you could move the above logic into a Cloud Function that's triggered by a delete request.