如何检查 Firebase 数据类 Android 中是否已存在值

Question

I want to check if the bus number already exists in the database of Firebase.

Here's my sample code. I've been searching for the past days but I can't find the right code to do so.

ref = new Firebase(Config.FIREBASE_URL);
postRef = ref.child("BusNumber");

busNum = edtBus.getText().toString().trim();
route1 = route.trim();
seat = edtSeat.getText().toString().trim();

if (!busNum.isEmpty() && !route1.isEmpty() && !seat.isEmpty()) {
    postRef.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            if (dataSnapshot.child(busNum).exists()) {
                edtBus.setError("Bus number already exists.");
                edtBus.setText("");
            } else {
                busNumber = new BusNumber();
                busNumber.setBusNum(busNum);
                busNumber.setRoute(route1);
                busNumber.setNumSeat(seat);
                postRef.push().setValue(busNumber);
                edtBus.setText("");
                edtSeat.setText("");
                Toast.makeText(AddBusActivity.this, "Saving successful!", Toast.LENGTH_SHORT).show();
            }
        }

        @Override
        public void onCancelled(FirebaseError firebaseError) {
            Toast.makeText(AddBusActivity.this, "Error", Toast.LENGTH_SHORT).show();
            Toast.makeText(AddBusActivity.this, firebaseError.getMessage(), Toast.LENGTH_SHORT).show();
        }
    });

} else {
    Toast.makeText(AddBusActivity.this, "Please complete the information", Toast.LENGTH_SHORT).show();
}

Can somebody help me with this matter? Thanks in advance. Whether the if statement is correct or not, also my problem is why does the postRef.addListenerForSingleValueEvent ...doesn't work? I tried to test some toast message but the message won't pop out.

Answer 1

Your approach is wrong.

When you are doing this dataSnapshot.child(busNum).exists() , it's looking for the busNum in the key section, where your keys are -kasajdh... .

So instead what you can do is, get the iterable , now when you look for data.child(busNum).exists() it relates to the value

   postRef.addListenerForSingleValueEvent(new ValueEventListener() {
                @Override
                public void onDataChange(DataSnapshot dataSnapshot) {
                for(DataSnapshot data: dataSnapshot.getChildren()){
                    if (data.child(busNum).exists()) {
                        //do ur stuff
                    } else {
                       //do something if not exists
                    }
                  }
                }

                @Override
                public void onCancelled(FirebaseError firebaseError) {

                }
            });

Answer 2

Rather than getting whole iterable list of data, you can query for exact entry.

  postRef = FirebaseDatabase.getInstance().getReference().child("BusNumber");

    postRef.orderByChild("busNum").equalTo(busNum)
        .addListenerForSingleValueEvent(new ValueEventListener() {

           @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                if(dataSnapshot.exists()){
                   //bus number exists in Database
            } else {
                //bus number doesn't exists.
            }

            @Override
            public void onCancelled(FirebaseError firebaseError) {

            }
        });

Answer 3

dataSnapshot.child(busNum).getValue() != null

应该工作。

Answer 4

It is difficult to guess the problem because you do not show how busNum and busNumber are defined and managed. Is busNumber a String?

push() creates a reference to an auto-generated child location. The auto-generated key looks something like -KPB_cS74yoDaKkM9CNB .

The statement postRef.push().setValue(busNumber) stores value busNumber in location BusNumber/<push-generated-key> .

The statement dataSnapshot.child(busNum).exists() tests for the existence of a value at location BusNumber/<busNum> . It will not be true unless busNum is one of the keys created by push() .

It's not clear how you want your data structured. If your bus numbers are Strings and are unique, you do not need to generate a key with push() . You could store the existence of bus numbers using:

postRef.child(busNumber).setValue(true)

Answer 5

if(!(dataSnapshot.child("Users").child(busNum).exists()))

然后是 hashmap 对象