"709894892396" : {
    "userComment" : {
      "comment" : "comment2",
      "dateComment" : 23423423143,
      "productId" : "892396",
      "ratting" : "5",
      "userId" : "709894",
      "userName" : "Jon"
    }
  }

db.ref("/").orderByChild("productId").equalTo("892396").on("value", function(snapshot) {
        console.log(snapshot.val());
        response.send(snapshot.key + " was " + snapshot.val()).end();
      }, function (errorObject) {
        response.send("The read failed: " + errorObject.code).end();
      });