Leave only NN newest entries and delete all the rest

Question

I have the following structure. Each user has his own data.

---+ root_child
   |
   +---+ Gy7FXRbRjDfAKWu7a95NgiGIZUk1  (Firebase User Id)
       |
       |
       +---+ KlNlb71qtQUXIGA4cNa (random key, generated by Firebase)
       |   |
       |   +--- timestamp = 1234567890
       |   |
       |   +--- (other data field ...)
       |
       |
       +---+ KlNlcmfMTDjxQ0BwW1K
       |   |
       |   +--- timestamp = 9876543211
       |   |
       |   +--- (other data field ...)
       |
       |
       +---+ (...)

Adding records occurs in this way:

databaseReference = FirebaseDatabase.getInstance().getReference("root_child");
databaseReference.child(firebaseUser.getUid())
    .push()
    .setValue(val);

push() ... Locations generated on a single client will be sorted in the order that they are created...

Now, how can I leave only 100 newest entries (for the specified user id) and delete all the rest?

Pseudocode :

databaseReference = FirebaseDatabase.getInstance()
    .getReference("root_child")
    .child(firebaseUser.getUid())
    .deleteLastNnRecords();

Answer 1

I would probably use the sort and filter functions.

First, I would grab the children count with a single value listener: https://stackoverflow.com/a/43607203/7949696 , Or in the case of a cloud function I would count the children from the snap on the write trigger.

Then, if childCount > 100, I would OrderByChild() on your timestamp and then filter by LimitToLast(childCount - 100)

If I were you, I would use cloud functions to achieve this and set a function on write to root_child/${uid}/

---

Edit: Source code link from comments: https://github.com/firebase/functions-samples/tree/master/limit-children

Cheers!

---

Edit 2: Looks like push() is chronologically sorted, so no need to OrderByChild

Answer 2

Algorithm:

1. AddListenerForSingleValueEvent is called only 1 time to get all the data and to delete all the old ones

2. Then we call only addChildEventListener to get only new data

       databaseReference.orderByKey()
           .addListenerForSingleValueEvent(valueEventListener =
                   new ValueEventListener() {
                       @Override
                       public void onDataChange(DataSnapshot dataSnapshot) {
                           long count = dataSnapshot.getChildrenCount();
                           long deleteFirst = count - Const.FB_MAX_CHILD;
                           long i = 0;
   
                           Log.i(TAG, "DataSnapshot children count = " + String.valueOf(count));
                           for (DataSnapshot data : dataSnapshot.getChildren()) {
                               i++;
                               if (i <= deleteFirst) {
                                   // delete
                                   Log.i(TAG, "delete => " + data.getKey());
                                   data.getRef().removeValue();
                               } else {
                                   TrackData trackData = data.getValue(TrackData.class);
                                   Log.i(TAG, data.getKey() + " = " + trackData.toString());
                               }
                           }
                       }