问如何使读取数据库后的代码在读取发生后发生

EN

  var count=0;
  var total=0;

  var ass_ref = admin.database().ref("modules/"+mod+"/Assignments");
  return ass_ref.once("value", function(snapshot) {

    snapshot.forEach(function(child) {

      var name = child.key;

      var ref = admin.database().ref("modules/"+mod+"/Assignments/"+name+"/"+uid);
      ref.once("value", function(mark) {

          count++;
          total=total+ parseInt(mark.val(),10);
          console.log("1:"+total);  //Value increases each time but occurs after 3

      });

      console.log("2:"+total);  //Value = 0 (occurs multiple times, in foreach)

    });

    console.log("3:"+total);  //Value = 0, logs once, after "2"