How to delete a specific child from a child in Firebase

Question

I have an application where a user can create lists.

The user can share a list with other users. I already manage to create the sharing part, the one I'm having issues is with the delete part. I want that when a user deletes a list that is shared , this list is also deleted from the other users.

This delete will be made only by list owner.

So a scenario will be:

• User A creates a List with pushID = 1.
• This list is added in the following firebase ref: /userList/$userAID/$pushID.
• User A shares list with User B and User C.
• This list is added in the following firebase ref: /userList/$userBID/$pushID and /userList/$userCID/$pushID.
• User A deletes list with pushID = 1.

So in my

So i have this schema:

userList: {
   2xYnKcZFEdPYWfUJ3E63yQEDShe2: {
     -Kt7lXiY0Yt-oDcV38L5
   }
   KtQHkXMSwKSByZ1rmTRwjDmSYnE3: {
     -Kt7lXiY0Yt-oDcV38L5: {}
     -Kt9XP91hjwcwgcBSgbc: {}
   }
   XHpMVoRqcCdzwTP70L29Lza1ibD3: {
     -Kt7lXiY0Yt-oDcV38L5: {}
   }
}

In high level this will be:

userList: {
       userID: (A) {
         -listID : (1) {}
       }
       userID: (B) {
         -listID: (1) {}
         -listID: (2) {}
       }
       userID: (C) {
         -listID: (1) {}
         -listID: (3) {}
         -listID: (4) {}
       }
    }

The current code I have to do this is the following:

const ref = firebase.database().ref('userList');
    ref.once('value')
      .then((snapshot) => {
        snapshot.forEach((childSnapshot) => {
          ref.child(childSnapshot.key)
            .once('value')
            .then((snapshot2) => {
              snapshot2.forEach((childSnapshot2) => {
                if (childSnapshot2.key === uid) {
                  ref.child(childSnapshot2.key)
                    .remove();
                }
              });
            })
            .catch(() => {
              console.log('error2');
            });
        });
      })
      .catch((error) => {
        console.log(error);
      });

What I'm doing in this code is, first fetching ALL the list inside userList, by getting the key I manage to jump to the userID child. Inside this node once again I manage to jump inside the pushID where I make a validation of checking if current key is equal to the UID of the list i want to delete, if so I do a remove().

I feel there must be a better way of telling Firebase to go directly to pushID and find all of those that are equal to the UID of the list I want to delete and then do it.

Answer 1

There is no way to do a server-side delete of multiple items based on a condition with the Firebase Database. You must first retrieve the items (or just their IDs) matching the condition and then delete those.

However, you can delete the list itself and all references to it in one go by using a multi-path update.

I'd also recommend keeping a list of all the UIDs you've shared a specific list with, so that you don't have to loop over all users. Keeping many-to-many relations in both directions is quite common in Firebase and other NoSQL databases.

For more see:

• the blog post introducing multi-path updates
• the blog post describing client-side fan-out using multi-path updates
• my answer on strategies for updating denormalized data
• my answer on modeling many-to-many relationships