如何在Firebase数据库中从子级检索特定数据

Question

I am new to Firebase database and my data structure in Firebase is like this:

Root- users-sec_a, sec_b, sec_c

In each section(sec_a, sec_b, sec_c) there will be uids of users. in each section there is min of 40 uids. And in all uids there is a common data child's (name , rollno , attendance,) Now my question was I want to display only names and rollno of all uids present in one of sections(section to be displayed was inputted by user).

How can I get that data?

Answer 1

it's fairly easy.

String sectionSelectedByUser = "sec_a"; // For example

Now getting refrence of the firebase database

if (FirebaseAuth.getInstance().getCurrentUser() != null){ // if you need user to be signed in.. 
            FirebaseDatabase.getInstance().getReference().child("users").child(sectionSelectedByUser).addListenerForSingleValueEvent(new ValueEventListener() {
                @Override
                public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                    for (DataSnapshot childSnapshot : dataSnapshot.getChildren()) { // iterates through all your UID in this section
                        if (childSnapshot.hasChild("name")){ // if current uid has name then fetch it
                            String name = childSnapshot.child("name").getValue().toString();
                        }

                        if (childSnapshot.hasChild("rollno")){ // if current UID has rollno then fetch it
                            String rollno =  childSnapshot.child("rollno").getValue().toString();
                        }
                    }
                }

                @Override
                public void onCancelled(@NonNull DatabaseError databaseError) {

                }
            });
        }

Answer 2

You can retrieve nested data with child method of Fire base database.

For you problem you can use this code with little modification as per requirements.

ValueEventListener singleEventListener = new ValueEventListener() {

    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        if (dataSnapshot != null) {
            Student student = dataSnapshot.getValue(Student.class);

            Log.d("TAG","name: "+student.getName());
        }
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
        Log.d("onCancelled - ", databaseError.toException());
    }
};

if (mFirebaseDatabaseReference == null) {
    mFirebaseDatabaseReference = FirebaseDatabase.getInstance().getReference();
    //mFirebaseDatabaseReference.keepSynced(true);
}
mFirebaseDatabaseReference = mFirebaseDatabaseReference.child(Constant_String.Table_User);
mFirebaseDatabaseReference.addListenerForSingleValueEvent(singleEventListener);

Hope this will help...

Answer 3

Assuming that your database schema looks like this:

Firebase-root
   |
   --- users
         |
         --- sec_a
         |    |
         |    --- uid
         |    |    |
         |    |    --- rollno: "Roll Number"
         |    |    |
         |    |    --- name: "User Name"
         |    |    |
         |    |    --- attendance: true
         |    |
         |    --- //other 40 users
         |
         --- sec_b
         |
         --- sec_c

And if you want for example to display only names and rollno of all uids present in one of sections (eg sec_a ), please use the following code:

DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference sectionRef = rootRef.child("users").child("sec_a");
ValueEventListener valueEventListener = new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        for(DataSnapshot ds : dataSnapshot.getChildren()) {
            String name = ds.child("name").getValue(String.class);
            String rollno = ds.child("rollno").getValue(String.class);
            Log.d(TAG, name + " / " + rollno);
        }
    }

    @Override
    public void onCancelled(@NonNull DatabaseError databaseError) {
        Log.d(TAG, databaseError.getMessage());
    }
};
sectionRef.addListenerForSingleValueEvent(valueEventListener);