fix(challenge export): do not fetch users and tasks concurrently

website/server/controllers/api-v3/challenges.js

@@ -591,21 +591,22 @@ api.exportChallengeCsv = {
     // computation on MongoDB but then iterated through all
     // results on the server so the perf difference isn't that big (hopefully)
 
-    const [members, tasks] = await Promise.all([
-      User.find({ challenges: challengeId })
-        .select(nameFields)
-        .sort({ _id: 1 })
-        .lean() // so we don't involve mongoose
-        .exec(),
+    const members = await User
+      .find({ challenges: challengeId })
+      .select(nameFields)
+      .sort({ _id: 1 })
+      .lean() // so we don't involve mongoose
+      .exec();
 
-      Tasks.Task.find({
+    const tasks = Tasks.Task
+      .find({
         'challenge.id': challengeId,
         userId: { $exists: true },
-      }).sort({ userId: 1, text: 1 })
-        .select('userId type text value notes streak')
-        .lean()
-        .exec(),
-    ]);
+      })
+      .sort({ userId: 1, text: 1 })
+      .select('userId type text value notes streak')
+      .lean() // so we don't involve mongoose
+      .exec();
 
     let resArray = members
       .map(member => [member._id, member.profile.name, member.auth.local.username]);