How to update an User with useMasterKey in Parse - parse-platform

Question

Issue Description
I'm trying to update an User when another user click on my Xamarin button.
Then, I used Cloud Code to perform this but it doesnt work
My Code
Here is my complete JS code :
Parse.Cloud.beforeSave("Archive", function(request, response) {
Parse.serverURL = 'https://pg-app-0brffxkawi8lqvf2eyc2isqrs66zsu.scalabl.cloud/1/';
var status = request.object.get("status");
if (status == "validated") {
var event = request.object.get("event");
event.fetch({
success: function(myEvent) {
var coinsEvent = myEvent.get("coins");
var user = request.object.get("user");
user.fetch({
success: function(myUser, coinsEvent, user) {
var email = myUser.get("email");
var coinsUser = myUser.get("coins");
myUser.set("coins", coinsUser + coinsEvent);
return myUser.save(null, {useMasterKey:true});
}
});
}
});
}
response.success();
});
I think myUser.save(null, {useMasterKey:true}); should work
I actually have that error :
Dec 24, 2017, 12:27 GMT+1 - ERRORError generating response for [PUT] /1/classes/_User/1GPcqmn6Hd
"Cannot modify user 1GPcqmn6Hd."
{
"coins": 250
}
Environment Setup
Server
parse-server version : v2.3.3
Server: Sashido

Answer 1

Your success branch never calls response.success() which is a problem... though maybe not THE problem.
You are also doing 2 fetches inside a 'beforeSave' function which is not recommended. 'BeforeSave' must happen very quickly and fetches take time. I would consider thinking through other options.
If you really need to do it this way, consider doing a Parse.Query("event") with an include("user") and trigger the query with query.first({useMasterKey:true}).
Are you sure coinsEvent is what you think it is? Fetch only returns the object fetched... not sure that you can curry in other parameters. I would change your final success routine to (double checking that coinsEvent is valid):
success: function(myUser) {
var coinsUser = myUser.get("coins");
myUser.set("coins", coinsUser + coinsEvent);
return myUser.save(null, {useMasterKey:true}).then(_ => response.success());
}