Android - Firebase complex (or not) query - Issue with unique IDs

Question

Today I'm experiencing an issue with Firebase to make a query such as: "Give me all the items of a user having that specific UID (user unique id)"

The items are added using the push method which creates a Unique ID. Is there some way to bypass the Unique ID for such a request?

Any help would be higly appreciated.

Thanks a lot in advance,

Firebase Structure:

Current code:

//New firebase instance
Firebase mFirebase = new Firebase(Constants.FIREBASE_ITEMS_URL);

//Query on records to find items related to users only
mQuery = mFirebase.child(uid).orderByChild(uid).equalTo(uid);

        mQuery.addChildEventListener(new ChildEventListener() {
            @Override
            public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                getUpdates(dataSnapshot);
            }

            @Override
            public void onChildChanged(DataSnapshot dataSnapshot, String s) {
                getUpdates(dataSnapshot);
            }

            @Override
            public void onChildRemoved(DataSnapshot dataSnapshot) {
                getUpdates(dataSnapshot);
            }

            @Override
            public void onChildMoved(DataSnapshot dataSnapshot, String s) {
                getUpdates(dataSnapshot);
            }

            @Override
            public void onCancelled(FirebaseError firebaseError) {
                Log.i(LOG_TAG, String.valueOf(firebaseError));
            }
        });

Answer 1

The best way I can see to accomplish "Give me all the items of a user having that specific UID (user unique id)" would be to modify your user firebase structure so it's as follows:

{
  users: {
    user1: {
      user_details { //whatever you need in here// }
      items: {
        -KHZlfskXT_aQtVOZytd: true,
        item1: true,
        item2: true,
        item3: true
      }
    }
  }
}

You can then query against that and get a list of all the itemIds for each user, then you can use that list of itemIds to retrieve the corresponding items -- To make a request with this layout: (i've written it in javascript)

var userItemsRef = mFirebase.child('users').child(uid).child('items')
var itemsRef = mFirebase.child('items');

userItemsRef.on('child_added', function(dataSnap) {
  itemsRef.child(dataSnap.key()).once('value', function(snap) {
    // do whatever you need to with the item
  });
});

For future reading on this type of thing, check out this article on denormalising your data in firebase

Answer 2

I'm definitely with Grant that it's probably better to store the items per user.

But as far as I can see, you should also be able to get the items in the current structure with:

String uid = "bd4dd...a670";
Firebase firebase = new Firebase("https://<app-name>.firebaseio.com");
Query query = firebase.child("items").orderByChild("uid").equalTo(uid);

Queries with Firebase sort-of "skip a level". So the above says: order all items by their uid and then give me the ones for the indicated user.

Answer 3

Beware that for some unknown reason, orderByChild("X").equalsTo("Y") will sometimes consistently return incomplete or null results.

When I ran into this problem, setPersistence(false) returned the correct results (I spent 2 hours trying to understand why Firebase was only returning a single result by uid value, in a list where all elements were using the same uid). However, when I restored the code to setPersistence(true), the problem returned.

So I setPersistence(false), uninstalled then reinstalled the app with setPersistence(true), and it fixed the problem (for now).

This is a serious issue, since having setPersistence(true) saves a lot of bandwidth. I'm still unsure how to fix it permanently.

Further reading: https://github.com/invertase/react-native-firebase/issues/13

UPDATE:

See this link for a better explanation and a solution: Firebase Offline Capabilities and addListenerForSingleValueEvent

I implemented the recommendations in the link, namely to use addValueEventListener instead of addListenerForSingleValueEvent, and it fixed the problem!

Still, if you're worried about Firebase making endless queries on the data instead of just a single query, just make sure to remove listeners when you got all your data or when you exit your fragment/activity, by using mDatabaseReference.removeEventListener([yourEventListener]).

Answer 4

FirebaseAuth mAuth = FirebaseAuth.getInstance();
    //getting the current user id
    String uid = mAuth.getCurrentUser().getUid();
    DatabaseReference dbroot = FirebaseDatabase.getInstance().getReference().getRoot();
    DatabaseReference items = dbroot.child("items");
    //firebase query
    Query  q = items.orderByChild("uid").equalTo(uid);