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Firebase onDataChange() method is not entered [duplicate]

I am trying to retrieve data from Firebase, but I don't know how. I have the following data structure in Firebase [1]: https://i.stack.imgur.com/vN7Ge.png:
![enter image description here][1]
I can retrieve the category title(sleep, Stress Relief, and Relax), but don't know how to retrieve the author.
dataSnapshot.child("author").getValue(String.class)) ; doesn't work.
.
databaseReference = FirebaseDatabase.getInstance().getReference().child("Category");
}
public void getDataFromFirebase() {
List<ParentItem> parentItemsList = new ArrayList<>();
databaseReference.addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot snapshot) {
for (DataSnapshot dataSnapshot : snapshot.getChildren()) {
String category = dataSnapshot.child("Category").getValue(String.class));
String author = dataSnapshot.child("author").getValue(String.class));
}
Log.d("TAG", "onDataChange: "+ parentItemsList);
}
UPDATE
now I have the following code to retrieve data, but onDataChange () is never reached.
list.add("Milena"); // this word shows in recyclerview
databaseReference = FirebaseDatabase.getInstance().getReference().child("Category");
databaseReference.addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
list.add("M");
for (DataSnapshot ds : dataSnapshot.getChildren()) {
String category = ds.child("Category").getValue(String.class);
list.add(category);
for (DataSnapshot data : dataSnapshot.getChildren()) {
author = data.child("author").getValue(String.class);
list.add(author);
}
}
}
firebase rules:
{
"rules": {
".read": "auth==true",
".write": "auth==true"
}
}
I have line list.add("Milena") to make sure the problem is with firebase, not the recycler view itself. Recycler view shows only the word "Milena". And I have line as the first line of OnDataChange method list.add("M"). The recycler view doesn't shows the letter.
I already added google-services.json file to my app and added this line to manifest <uses-permission android:name="android.permission.INTERNET"/>
Why I can't still retrieve data?

If you want get field author, you can more loop for dataSnapshot like this
for (DataSnapshot dataSnapshot : snapshot.getChildren()) {
String category = dataSnapshot.child("Category").getValue(String.class));
for(DataSnapshot data : dataSnapshot.getChildren()) {
String author = data.child("author").getValue(String.class));
}
}
hope this can solve your problem :)

I think that the problem is your reference points to Category and you want the author. Between category and authors there are 2 children ("Sleep" and "1").
So, I think the best to do is change this line:
databaseReference = FirebaseDatabase.getInstance().getReference().child("Category");
in this:
databaseReference = FirebaseDatabase.getInstance().getReference().child("Category").child("Sleep").child("1");

To solve this issue you have to iterate through the children twice, as seen in the following lines of code:
DatabaseReference db = FirebaseDatabase.getInstance().getReference();
DatabaseReference categoryRef = db.child("Category");
categoryRef.get().addOnCompleteListener(new OnCompleteListener<DataSnapshot>() {
#Override
public void onComplete(#NonNull Task<DataSnapshot> task) {
if (task.isSuccessful()) {
DataSnapshot snapshot = task.getResult();
for (DataSnapshot categorySnapshot : snapshot.getChildren()) {
String categoryName = categorySnapshot.getKey();
Log.d("TAG", categoryName);
for (DataSnapshot ds : categorySnapshot.getChildren()) {
String author = ds.child("author").getValue(String.class);
Log.d("TAG", author);
}
}
} else {
Log.d("TAG", task.getException().getMessage()); //Never ignore potential errors!
}
}
});
But be aware that a child as Category: "Sleep" should not exist on the same level as 1, 2, and so on. Since the name of the category already exists as the key of a node, please remove those records:
In this way, you'll avoid a ClassCastException, as at the same level you have an object with two properties and a String one. So once you remove those children, the above code will work perfectly fine. If you however need a node to contain that information, I recommend you create a top-level node that will contain only the name of the categories.

How to get the key of a pushed value

In Firebase Realtime Database on Android, I want to retrieve this id "-Mb1sSv-FCNr9ElxZIwN".
databaseReference = FirebaseDatabase.getInstance().getReference().child("Registration Data1");
mDBListener = databaseReference.orderByKey().limitToLast(1).addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot dataSnapshot) {
mTeachers.clear();
for (DataSnapshot teacherSnapshot : dataSnapshot.getChildren()) {
Data DB = teacherSnapshot.getValue(Data.class);
DB.setKey(teacherSnapshot.getKey());
mTeachers.add(DB);
}
mAdapter.notifyDataSetChanged();
mProgressBar.setVisibility(View.GONE);
}

It looks to me that KAsr ... dhz1 is the user ID that comes from the authentication process. If so, that UID should also be added to your reference. So to be able to get this id "-Mb1sSv-FCNr9ElxZIwN", please change the following reference:
databaseReference = FirebaseDatabase.getInstance().getReference().child("Registration Data1");
To:
String uid = FirebaseAuth.getInstance().getCurrentUser().getUid();
databaseReference = FirebaseDatabase.getInstance().getReference().child("Registration Data1").child(uid);
Meaning that:
DB.setKey(teacherSnapshot.getKey());
Will set the above key to the "DB" object.

How to remove one Item in the database of the fire base?

How to remove one Item in the database of the fire base?
I have code to delete the entire database:
DatabaseReference ref = FirebaseDatabase.getInstance().getReference();
Query applesQuery = ref.child("Markers").orderByChild(firebaseAuth.getUid());
applesQuery.addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot appleSnapshot : dataSnapshot.getChildren()) {
appleSnapshot.getRef().removeValue();
}}
#Override
public void onCancelled(DatabaseError databaseError) {
Log.e(TAG, "onCancelled", databaseError.toException());
}
});
the picture shows that in the Marker database there are 2 entries and they have folded entries !!! How to delete one entry (logged in user), and the second should remain in the database ???

As those keys are user id's you can do the following
FirebaseDatabase.getInstance().getReference()
.child("Markers")
.child(firebaseAuth.getUid())
.removeVaule();
It will delete that node and folded data too

Change the query to the following :
Query appQuery = ref.child("Markers").orderByKey().equalTo(firebaseAuth.getUid());

Why does my firebase database child return null?

I am working with firebase database and i was trying to get the value of a child using this code below and may System.out.println(MyCredit.toString()); is always returns null: Please see my firebase database
DatabaseReference ref = FirebaseDatabase.getInstance().getReference().child("Users");
ref.addListenerForSingleValueEvent(
new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
//Get map of users in datasnapshot
CollectCredits((Map<String,Object>) dataSnapshot.getValue());
}
#Override
public void onCancelled(DatabaseError databaseError) {
//handle databaseError
}
});
private void CollectCredits(Map<String,Object> users) {
MyCredit = new ArrayList<>();
//iterate through each user, ignoring their UID
for (Map.Entry<String, Object> entry : users.entrySet()){
//Get user map
Map singleUser = (Map) entry.getValue();
//Get Credit field and append to list
MyCredit.add((Long) singleUser.get("Credit"));
Toast.makeText(MainActivityCustonlistViewnew.this, "Credit : " +String.valueOf(MyCredit), Toast.LENGTH_SHORT).show();
}
System.out.println(MyCredit.toString());
}

Try this:
private void CollectCredits(DataSnapshot dataSnapshot) {
MyCredit = new ArrayList<>();
//iterate through each dataSnapshot
for (DataSnapshot d1 : dataSnapshot.getChildren()){
MyCredit.add(d1.getValue());
}
}

Let me give you some suggestions, Try to check your firebase database path both the root and child path because some time you did everything correctly but the path you specified to your Database reference which is not referring to the correct path. So it can produce the same result what are getting right now.

Android Firebase Fetch Data Of Every User

For example, there are 2 main nodes in my database (there can be more). The parent node is the user's authentication id and inside it
there is some detail as you can see.
I am trying to do that if any user login all other user's data populated on his activity
but I really don't know how to start because every user has its own authentication id and there are sub-nodes also like map location and image
How can I fetch every user's detail to the activity?

What you can do is that get the data from the fbuserinfo node using an event listener like this and iterate through the data. Here User.class is the class for the User model that you have created
DatabaseReference userDataReference = FirebaseDatabase.getInstance().getReference()
.child("fbuserinfo");
ValueEventListener valueEventListener = new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot ds : dataSnapshot.getChildren()) {
User user = ds.getValue(User.class);
Log.d("User", user);
}
}
}
#Override
public void onCancelled (DatabaseError databaseError){
Log.d(TAG, "Error fetching user data - " + databaseError.getMessage());
}
}
userDataReference.addListenerForSingleValueEvent(eventListener);
And then if you want to display the data, you can do it in the for loop and have an if condition to check if the user is not equal to the current one

Using the following example you'll be able to log the name of all users from your database.
DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference fbRef = rootRef.child("Users").child("fbusersinfo");
ValueEventListener eventListener = new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
for(DataSnapshot ds : dataSnapshot.getChildren()) {
String name = ds.child("name").getValue(String.class);
Log.d("TAG", name);
}
}
#Override
public void onCancelled(DatabaseError databaseError) {}
};
fbRef.addListenerForSingleValueEvent(eventListener);
The output will be:
Ahmend Abbas
mahanoor 4
If you need also the other properties, you can get them in the same manner.