Checking for a string in firestore always returns true

Question

I am new to android development and firestore. What I am trying to do is compare whether a string exists in the database or not. However, the query always returns true even if I am trying to compare with string that is not present in the database. Here is my code:

public void loadData(View v){
        db.collection("Machines")
                .whereEqualTo("machineId","AYSLE004")
                .get()
                .addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
                    @Override
                    public void onSuccess(QuerySnapshot queryDocumentSnapshots) {
                        //MachineId exists
                        Toast.makeText(MainActivity.this, "Machine Id exists", Toast.LENGTH_LONG).show();
                    }
                })
                .addOnFailureListener(new OnFailureListener() {
                    @Override
                    public void onFailure(@NonNull Exception e) {
                        //Machine Id does not exist
                        Toast.makeText(MainActivity.this, "Machine Id does not exist", Toast.LENGTH_LONG).show();
                        Log.d(TAG,e.toString());
                    }
                });
    }

The string "AYSLE004" does not exist in the database. However the toast message shows "Machine Id exists". Please assist me where I am going wrong. Thanks in advance.

Answer 1

尝试添加以下内容：

.addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
    @Override
    public void onSuccess(QuerySnapshot queryDocumentSnapshots) {
        //MachineId exists
        if(queryDocumentSnapshots != null && !queryDocumentSnapshots.isEmpty()) {
            Toast.makeText(MainActivity.this, "Machine Id exists", Toast.LENGTH_LONG).show();
        } else {
            //it doesn't exist
        }
    }
}

这样做的原因（我认为）是因为onSuccess调用Firebase是因为对Firebase的请求已完成并实现successfully，只是没有找到您的请求。