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Retrieving data realtime from firebase that have 2 childs for 2 apps in 1 firebase [Android]

I create 2 android apps, for the user and for admin. The user app can crud the "pelanggaran", the admin app can see all the pelanggaran realtime.
Here, I am stuck in retrieving the data in real time for the admin app. Here's the database hierarchy.
this is the references: pelanggaran and user
each user has pelanggaran. so, the first child of pelanggaran is userID, then the pelanggaranID, then the values
I tried googling, I got it, but it is not real-time. Here's my code
private void refreshList(){
databaseUser = FirebaseDatabase.getInstance().getReference("User");
pelanggaranList = new ArrayList<>();
databaseUser.addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
pelanggaranList.clear();
long numberUsers = dataSnapshot.getChildrenCount();
Log.w("total user", "" + numberUsers);
for (DataSnapshot dsUser : dataSnapshot.getChildren()){
User user = dsUser.getValue(User.class);
final String userId = user.getUserId();
Log.w("id user", userId);
databasePelanggaran = FirebaseDatabase.getInstance().getReference("Pelanggaran").child(userId);
databasePelanggaran.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot1) {
for (DataSnapshot dsPelanggaran : dataSnapshot1.getChildren()){
Pelanggaran pelanggaran = dsPelanggaran.getValue(Pelanggaran.class);
Log.w("id pelanggaran", userId + " : " + pelanggaran.getIdPelanggaran());
pelanggaranList.add(pelanggaran);
}
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
}
PelanggaranAdapter pelanggaranAdapter = new PelanggaranAdapter(MainActivity.this, pelanggaranList);
recyclerView.setAdapter(pelanggaranAdapter);
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
}
It can take all the values from each user but it is not real time. And sometimes the data doesn't appears. What should I do to make it realtime?

Android Retrieving Single Value from FireBase-RealTime Database

Answer needed urgently. I'm trying to retrieve a single value school name as marked in the snapshot attached below and populate it to my android spinner from my firebase-database
private class FireBaseConnection{
FirebaseDatabase database = FirebaseDatabase.getInstance();
DatabaseReference rootRef = database.getReference().child("root");
DatabaseReference reg_schools = rootRef.child("reg_schools");
}
private ArrayList<String> retrieveAllSchools(){
pBar.setMessage("Please Wait..Retrieving Schools...");
pBar.show();
FireBaseConnection fireBaseConnection = new FireBaseConnection();
String key = fireBaseConnection.reg_schools.push().getKey();
fireBaseConnection.reg_schools.child(key).child("school_name").addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
for(DataSnapshot snapshot : dataSnapshot.getChildren()){
resultProcessorObject = snapshot.getValue(SchoolObject.class);
schools_retrieved = resultProcessorObject.getSchool_name();
schools.add( schools_retrieved);
//schools = (ArrayList<Object>) snapshot.getValue();
// Toast.makeText(Main2Activity.this, "Schools are : " + schools, Toast.LENGTH_SHORT).show();
}
Toast.makeText(Main2Activity.this, "Schools are : " + schools, Toast.LENGTH_SHORT).show();
pBar.dismiss();
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
return schools;
}

So I want to answer my question..I made it work with the help of #Alex Mamo video
so everything remained unchanged..all i did was tweak my code a bit
Instead of
fireBaseConnection.reg_schools.child(key).child("school_name").addValueEventListener(){
//Then my codes here
}
Instead i did
fireBaseConnection.reg_schools.addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
for(DataSnapshot snapshot : dataSnapshot.getChildren()){
resultProcessorObject = snapshot.child("school_name").getValue(String.class);
//schools_retreived = resultProcessorObject.getSchool_name();
schools.add(resultProcessorObject);
count = dataSnapshot.getChildrenCount();
Toast.makeText(Main2Activity.this, "Total number of " + count + " schools were retreived ", Toast.LENGTH_SHORT).show();
// schools = (ArrayList<Object>) snapshot.getValue();
// Toast.makeText(Main2Activity.this, "Schools are : " + schools, Toast.LENGTH_SHORT).show();
}
Toast.makeText(Main2Activity.this, "Schools are : " + schools, Toast.LENGTH_SHORT).show();
pBar.dismiss();
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
So Basically i used the snapshot to reference the node "school_name" i'm looking for in the onDataChange Method

Create variable only if Firebase database does not exist

I am creating an application that starts the home activity and creates the methods in the Firebase database. This does it correctly but each user has a variable called acoins and I want this every time someone enters if it already exists that is not set to 0 that stays with the value that has.
This is the code I have :
databaseRef.child("users/" + user.getDisplayName() + "/acoins").setValue(0);
­
private void setUserData(FirebaseUser user) {
nameTextView.setText(user.getDisplayName());
emailTextView.setText(user.getEmail());
idTextView.setText(user.getUid());
Glide.with(this).load(user.getPhotoUrl()).into(photoImageVie‌​w);
DatabaseReference databaseRef = FirebaseDatabase.getInstance().getReference();
databaseRef.child("users/" + user.getDisplayName()).setValue(user.getEmail());
databaseRef.child("users/" + user.getDisplayName() + "/acoins").setValue(0);
}

Try this in setUserData() method
DatabaseReference databaseRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference dr = ref.child("users").child(user.getDisplayName).child("acoins");
dr.addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
Long acoins1 = (Long)dataSnapshot.getValue();
if(acoins1 != 0)
{//create toast or anything here
}
else{
databaseRef.child("users/" + user.getDisplayName() + "/acoins").setValue(0);
}
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});

How to get a specific item inside the key using android

How to only retrieve email addresses inside the key, not all the items. I have an items Name and Email, but I only want to retrieve EMAIL.
example:
Email: HOTMAIL
Email: GMAIL

first make database reference as
mReference= FirebaseDatabase.getInstance().getReferenceFromUrl(FIREBASE_URL_USERS);
here FREBASE_URL_USERS is a url of users key you can get it from firebase console.
it is a combination of your unique_fiirebase_root_url / User01
then get snapshot of data as
mDatabseReference.addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
dataSnapshot.getValue("Email");
// use log to check whetehr you are getting email or not
#Override
public void onCancelled(DatabaseError databaseError) {
// log error and do something
}
});

Assuming that the user emails are unique, you can query the users records by its email value as follows:
final String email = "hani.mehdi#email.com";
ValueEventListener onFindListener = new ValueEventListener() {
#Override
public void onCancelled(DatabaseError error) {
Log.e(TAG, error.getMessage());
}
#Override
public void onDataChange(DataSnapshot snapshot) {
Map<String, User> result = snapshot.getValue(Map.class);
if (result.contains(email) {
User user = result.get(email);
Log.i(TAG, user.toString());
} else {
Log.d(TAG, "Not found: " + email);
}
}
};
DatabaseReference userReference = database.getReference("users")
.orderByChild("email")
.equalTo(email)
.addListenerForSingleValueEvent(onFindListener);

How to retrieve data and specific user data from the Firebase

I am having problem in retrieving data from firebase. I have one root user's node and then inside that root I have one userID child node. Inside that child node I am storing user information according to its blood group.
Now, what I want is to fetch the name and email according to blood group, say fetch all user data whose blood group is B+.
Also, please tell me on how to fetch all entries in the firebase and show on textView.
//show data on text view
datashow.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
Log.i("inside click", "onClick: ");
FirebaseDatabase database = FirebaseDatabase.getInstance();
DatabaseReference db = database.getReference();
db.child("users").addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
User user = dataSnapshot.getValue(User.class);
if (user == null) {
Log.e(TAG, "User data is null!");
return;
}
Iterable<DataSnapshot > children = dataSnapshot.getChildren();
Log.i("children", "onDataChange: " + children);
for(DataSnapshot child : children)
{
Map<String , String> map = (Map)child.getValue();
Log.i("inside", "onDataChange: " + map);
name = map.get("name");
email = map.get("email");
dateof = map.get("userDob");
showData(name , email , dateof );
Log.i("datasnap", "onDataChange: data snapshot " + name + email + dateof);
}
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
}
});
this is my snapshot for Firebase database
enter image description here

db.child("users").orderByChild("userBloodGroup").equalTo("B+").addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot snapshot) {
List<User> data = new ArrayList<>();
if (dataSnapshot.exists()) {
for (DataSnapshot snapshot :
dataSnapshot.getChildren()) {
User element = snapshot.getValue(User.class);
element.setKey(snapshot.getKey());
data.add(element);
}
for (User user: data){
Log.i(TAG,"user name: " +user.getName());
Log.i(TAG,"user email: " +user.getEmail());
Log.i(TAG,"user dob: " +user.getUserDob());
}
}
#Override
public void onCancelled(DatabaseError databaseError) {
Log.i(TAG, "loadPost:onCancelled", databaseError.toException());
}
});

To query for all users with the bloodGroup B+, use a Firebase Query:
firebaseReference("users").orderByChild("userBloodGroup").equalTo("B+")
To get all the data of the users, you could use a Listener to get all of them and put them in whatever TextView you need. You could check the documentation on Firebase on how they read data. https://firebase.google.com/docs/database/android/read-and-write