Firebase Realtime Database comparing value inside the child notes with another child note

Question

I have a database something like this. How I want to compare the value for all users to get most value.

restaurant
    -userUid
        -stateUid
            -restaurantUid
                -price = 9
            -restaurantUid2
                -price = 10
        -stateUid2
            -restaurantUid3
                -price = 2

As you can see the database there, stateUid price is 19 while stateUid2 price is only 2 So, stateUid has the most price. How to compare them and display the most one. Thank you

EDIT:

I have done something like this, and it's error at return. And the value is not working.

exports.calculateTotal = functions.database.ref('/restaurant/{userUid}/{stateUid}/{restaurantUid}')
    .onWrite((change, context) => {
        // Only edit data when it is first created.
        if (change.before.exists()) {
            return null;
        }
        // Exit when the data is deleted.
        if (!change.after.exists()) {
            return null;
        }

        //Get id
        const restaurantUid = context.params.restaurantUid;

        let totalValue = 0;
        change.after.forEach(function (item) {
            totalValue += item.child('total').val();
        });
        console.log(totalValue);
        return functions.database.ref('/priceTotal/' + restaurantUid).child('total').set(totalValue);
    });

Do you want to display your dataset somewhere like recyclerview or just want to get the highest price state ? — 50_Seconds _Of_Coding, Jun 27 '20 at 13:10
I want just to get the highest price state @50_Seconds_Of_Coding — Ticherhaz FreePalestine, Jun 27 '20 at 13:17
You edit makes this a completely different question, as it's no longer about Android. This means both existing answers now make little sense for the question. — Frank van Puffelen, Jun 27 '20 at 14:52
From your comment on my answer it seems you get an error within the Cloud Functions code. Please edit your question to include the complete error and stack trace. — Frank van Puffelen, Jun 27 '20 at 14:52
@FrankvanPuffelen nvm, I've made up to make new node to store them separetely. Thank you sir — Ticherhaz FreePalestine, Jun 28 '20 at 07:54

score 1 · Accepted Answer · answered Jun 27 '20 at 13:40

Firebase queries work on a flat list of nodes. A query can contain only a single unknown key, the key of the direct child nodes under the location where you query. In your data structure there are multiple levels of unknown keys, which means that you can't query for the highest price across all of them.

What you can do in your current data structure is query across one state for the restaurant with the highest price. That'd look something like:

DatabaseReference ref = FirebaseDatabase.getInstance().getReference("restaurant");
DatabaseReference stateRef = ref.child("userUid").child("stateId");
stateRef.orderByChild("price").limitToLast(1).addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        for (DataSnapshot snapshot: dataSnapshot.getChildren()) {
            Log.i(TAG, snapshot.getKey()+": "+snapshot.child("price").getValue(Long.class));
        }
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
        throw databaseError.toException();
    }
}

But you can't search across all states for a user, or even all users. If you want to allow that, you'll have to store all prices in a flat list, like:

restaurant_prices: {
  "restaurantid": {
    price: 9,
    state: "statid",
    user: "userid"
  }
}

Also see:

Hi Frank, thanks for answering, if using Cloud functions can solve this situation? event `onWrite`, after that, create new nodes `totalPrice/restaurantId/total`. — Ticherhaz FreePalestine, Jun 27 '20 at 13:46
If you mean whether you can use Cloud Functions to create the additional data structure needed for this query, then that is indeed an option. — Frank van Puffelen, Jun 27 '20 at 13:53
I've edited the question, I did search about this https://stackoverflow.com/a/51436573/9346054 , but still not working — Ticherhaz FreePalestine, Jun 27 '20 at 14:29

50_Seconds _Of_Coding · Answer 2 · 2020-06-27T13:40:05.730

int totalPrice = 0;
int greaterPrice = 0;
int temp = 0;

DatabaseRefernce restRef = FirebaseDatabase.getInstance().getReference().child("restaurant").child(userUid);

restRef.addValueEventListener(new ValueEventListener(){
       
    onDataChange(Datasnapshot snapshot) {
        
        for(Datasnapshot snap : snapshot) {
          
            String key = snap.getKey();
            //This will return you the keys of stateUid
            restRef.child(key).addValueEventListener(new ValueEventListener(){
                   
                   onDataChanged(DatSnapshot datasnapshot) {
    
                           //this for loop will iterate through restaurants of that specific state
                           for(DataSnapshot snap2 : datasnapshot){
                           
                                 totalPrice += (int) snap2..child("price").getValue();
                           
                           }
                           //When this loop ends you will get the total price of all restaurants from that state

                   }
                    
        
            });

           //as u see above I mentioned greater price and temp variable
           using simple logic of finding greatest number out of two number save the value of greatest integer to the variable every time you loop through state

        } 

    }

  }
);

Use nested for loops to iterate from database like above and calculate your prices

Else what you can do is when you are uploading the data of restos - while uploading prices just make an extra node for total price of city and add price of resto every time you upload new resto

Firebase Realtime Database comparing value inside the child notes with another child note

2 Answers2