Retrieve an Observable (including key) using RxFirebaseDatabase

ObservableListData.java

        RxFirebaseDatabase
                .observeValueEvent(database.getReference("list"))
                .map(new Function<DataSnapshot, List<DataEntity>>() {
                    @Override
                    public List<DataEntity> apply(@NonNull DataSnapshot parent) throws Exception {
                        List<DataEntity> list = new ArrayList<>();
                        for (DataSnapshot item : parent.getChildren()) {
                            DataEntity entity = new DataEntity();
                            entity.setEndDate((Integer) item.child("endDate").getValue());
                            entity.setStartDate((Integer) item.child("startDate").getValue());
                            entity.setKey(item.getKey());
                            list.add(entity);
                        }
                        return list;
                    }
                })
                .toObservable();