I am new to firebase
and nosql
database
I am trying to retrieve some specific data from a Firebase. I have a node universities
and that node has many unique identifier as node which has so more data into it. I want to retrieve name
from each node.
Please have a look at this.
What I have tried so far: I tried using addChildEventListener
but this only listens first child. I've debugged it and it was only showing the values of the first child of universities
node.
myRef.addChildEventListener(new ChildEventListener() {
@Override
public void onChildAdded(DataSnapshot dataSnapshot, String s) {
List<University> university = new ArrayList<>();
Map<String, Object> td = (HashMap<String, Object>) dataSnapshot.getValue();
Log.d("TAG",dataSnapshot.getValue().toString());
}
@Override
public void onChildChanged(DataSnapshot dataSnapshot, String s) {
}
@Override
public void onChildRemoved(DataSnapshot dataSnapshot) {
}
@Override
public void onChildMoved(DataSnapshot dataSnapshot, String s) {
}
@Override
public void onCancelled(DatabaseError databaseError) {
}
});
I have also tried addValueEventListener
. This listens the whole node and return whole data, but I am unable to extract "name" for it since it contains unique identifiers.
Please guide me into the right directions.
myRef.addValueEventListener(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
University university = dataSnapshot.getValue(University.class);
List<University> universities = (List<University>) dataSnapshot.getValue(University.class);
*//*List<String> lst = new ArrayList<String>();
for(DataSnapshot dsp : dataSnapshot.getChildren()){
lst.add(dsp.getKey());
}
Log.d("TAG",lst.toString());*//*
Map<String, Object> td = (HashMap<String, Object>) dataSnapshot.getValue();
*//*List<Object> values = new ArrayList<Object>(td.values());
List<String> list=new ArrayList<String>();
for(int i=0; i<values.size(); i++){
list.add((String) values.get(i));
}*//*
Log.d("TAG", university.toString());
}
@Override
public void onCancelled(DatabaseError databaseError) {
}
});
Firebase data is retrieved by either a one time call to GetValueAsync() or attaching to an event on a FirebaseDatabase reference. The event listener is called once for the initial state of the data and again anytime the data changes.
We can filter data in one of three ways: by child key, by key, or by value. A query starts with one of these parameters, and then must be combined with one or more of the following parameters: startAt , endAt , limitToFirst , limitToLast , or equalTo .
If you using addValueEventListener
for retrieve all university from Firebase
List<University> universityList = new ArrayList<>();
myRef.addValueEventListener(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot snapshot) {
universityList.clear();
for (DataSnapshot postSnapshot: snapshot.getChildren()) {
University university = postSnapshot.getValue(University.class);
universityList.add(university);
// here you can access to name property like university.name
}
}
@Override
public void onCancelled(DatabaseError databaseError) {
System.out.println("The read failed: " + firebaseError.getMessage());
}
});
Or if you using addChildEventListener
List<University> universityList = new ArrayList<>();
myRef.addChildEventListener(new ChildEventListener() {
@Override
public void onChildAdded(DataSnapshot dataSnapshot, String s) {
University university = dataSnapshot.getValue(University.class);
universityList.add(university);
Log.i(TAG,"add university name = " + university.name);
...
}
...
}
You can use equalTo() of Query in which you can pass your custom string to match with firebase database.
like this way you can create your Query and set listener with that
Query queryRef = myFirebaseRef.orderByChild("name").equalTo("some name");
refer this thread for more.
Here is nice example from firebase officials, refer that and you will get clear idea
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