在嵌套数据库firebase android中获取最后一项

Question

这是我的数据结构:

在这里,我想根据id ="sd"检索Location子项的最后一项的纬度和经度.

这是代码:

Query query = mDatabase.child("Users").orderByChild("Id").equalTo(busId);
// busId = "sd"
query.addListenerForSingleValueEvent(new ValueEventListener() {
                @Override
                public void onDataChange(DataSnapshot dataSnapshot) {
                    for(DataSnapshot user : dataSnapshot.getChildren()) {
                        Query lastItem = user.getRef().child("Location").orderByKey().limitToLast(1);
                        lastItem.addListenerForSingleValueEvent(new ValueEventListener() {
                            @Override
                            public void onDataChange(DataSnapshot dataSnapshot) {
                                String lat = dataSnapshot.child("Latitude").getValue().toString();
                            }

                            @Override
                            public void onCancelled(DatabaseError databaseError) {

                            }
                        });
                        break;
                    }
                }

我怎样才能做到这一点？谢谢

Answer 1

Android 版请使用此代码：

DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference userRef = rootRef.child("Users");
ValueEventListener eventListener = new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        for(DataSnapshot ds : dataSnapshot.getChildren()) {
            String Id = ds.child("Id").getValue(String.class);
            if(Id.equals("sd")) {
                Query q = ds.child("Location").getRef().orderByKey().limitToLast(1);

                ValueEventListener valueEventListener = new ValueEventListener() {
                    @Override
                    public void onDataChange(DataSnapshot dataSnapshot) {
                        for(DataSnapshot child : dataSnapshot.getChildren()) {
                            Double latitude = child.child("Latitude").getValue(Double.class);
                            Double longitude = child.child("Longitude").getValue(Double.class);
                            Log.d("TAG", latitude + " / " + longitude);
                        }
                    }

                    @Override
                    public void onCancelled(DatabaseError databaseError) {}
                };
                q.addListenerForSingleValueEvent(valueEventListener);
            }
        }
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {}
};
userRef.addListenerForSingleValueEvent(eventListener);