Leave Only Nn Newest Entries And Delete All The Rest

I have the following structure. Each user has his own data. ---+ root_child | +---+ Gy7FXRbRjDfAKWu7a95NgiGIZUk1 (Firebase User Id) | | +---+ KlNlb71qtQ

Solution 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}/

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

Cheers!

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

Solution 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 =
                   newValueEventListener() {
                       @OverridepublicvoidonDataChange(DataSnapshot dataSnapshot) {
                           longcount= dataSnapshot.getChildrenCount();
                           longdeleteFirst= count - Const.FB_MAX_CHILD;
                           longi=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 {
                                   TrackDatatrackData= data.getValue(TrackData.class);
                                   Log.i(TAG, data.getKey() + " = " + trackData.toString());
                               }
                           }
                       }