#[post("/search_username")]
pub(crate) async fn query_user(
    mongo_conn: Data<Client>, 
    form: web::Form<Username>
) -> HttpResponse { 
    let username = &form.username;
    let collection = mongo_conn.database("ngiam-db").collection("userEntity");
    let filter = doc! {"username":username};
    let fo = FindOptions::builder()    
        .projection(doc! {"username":1, "displayName":1, "groupChain":1,      
        "_id":0}) .build(); 
    let cursor = collection.find(filter, fo).await;    <--- this line
    
    unimplemented!()
}