从Firebase中的父级检索第一个孩子

Question

我正在开发一个Android应用程序。我想要父'支持'的第一个子（第一个UID）并将其存储在String变量中。如何从列表中获取第一个UID的值？我试过一种方法。它不起作用。

@Override
public void onDataChange(DataSnapshot dataSnapshot1) {
    if (dataSnapshot1.exists()) {
        String futureUID = "";
        for(DataSnapshot futureUIDdatasnapshot:dataSnapshot1.getChildren() ){
            futureUID = futureUIDdatasnapshot.getKey();
            break;
        }
        /*Getting the first UID from the list of UID's in queue in 'future'*/
        futureUID = dataSnapshot1.getChildren().iterator().next().getKey();
        /*Moving a card from 'future' to 'serving'*/
        societyServiceUIDReference.child(FIREBASE_CHILD_SERVING).child(futureUID).setValue(FIREBASE_ACCEPTED);
        /*Removing the UID from 'future' after it is placed in 'serving'*/
        societyServiceUIDReference.child(FIREBASE_CHILD_FUTURE).child(futureUID).removeValue();
    }

}

@Override
public void onCancelled(DatabaseError databaseError) {

}

注意：'futureUID'是我想要的UID

Answer 1

请尝试以下方法：

DatabaseReference ref=FirebaseDatabase.getInstance().getReference().child("support");
Query queryUid=ref.orderByKey().limitToFirst(1);
queryUid.addListenerForSingleValueEvent(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot datas : dataSnapshot.getChildren()) {
        String key=datas.getKey();
            }
        }

 @Override
public void onCancelled(DatabaseError databaseError) {

    }
});

Answer 2

尝试

DatabaseReference mDatabase;
mDatabase = FirebaseDatabase.getInstance().getReference();

.orderByKey（）。limitToFirst（n）是什么诀窍。它按键对查询结果进行排序，只返回前n个结果;在这种情况下1

mDatabase.getChild("support").orderByKey().limitToFirst(1)
.addListenerForSingleValueEvent(new ValueEventListener () {

      @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
          if(dataSnapshot.exists()){
              for (DataSnapshot supportItem: dataSnapshot.getChildren()) {
              String futureUID =supportItem.getKey();
            }
        }

    }

    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
         //Catch your error here
    }
});

见Work with Lists of Data on Android