Firebase检查是否存在返回false，它应该返回true

Question

I'm trying to check if a user with a particular email exists or not. The user with this email is in the database so the check should have returned true instead of false.

Here's the schema:

Here's the code:

final DatabaseReference userRef = FirebaseDatabase.getInstance().getReference().child("users");
userRef.orderByChild("email").equalTo("rafael.adel20@gmail.com").limitToFirst(1).addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        System.out.println(dataSnapshot); //returns { key = users, value = null }
        if(dataSnapshot.getChildrenCount() > 0) {
            //user already exists (Never executed)
        } else {
            //user doesn't exists (Always executed)
        }
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {

    }
});

Answer 1

If setPersistenceEnabled(true) is called, use of addListenerForSingleValueEvent can cause local cached version of data to be returned rather than latest version from server (whereas calling addValueEventListener will ensure you get latest version). Another approach, if using addListenerForSingleValueEvent is to call query.keepSynced(true);