 
         , ->
 
+            updater =
+                $set: {}
+
+            updater.$set["timestamp#{type}"] = timestampMax
+
             Database.db.collection('chat_data').update
                 _id: 'last_task'
-            ,    
-                $set:
-                    'timestamp_' + type: timestampMax
+            , updater
             ,
                 upsert: true
             , (err) ->

        if response.length < Config.Chat.FetchItemCount

            # no more messages: remove task
            
            delete Chat.tasks[taskId]
            Chat.length--


                    minLngE6:        Math.round(Config.Region.SouthWest.Lng * 1e6)
                    maxLatE6:        Math.round(Config.Region.NorthEast.Lat * 1e6)
                    maxLngE6:        Math.round(Config.Region.NorthEast.Lng * 1e6)
                    minTimestampMs:  TSmin - 1000
                    maxTimestampMs:  TSmax + 1000
                    chatTab:         type
                status: STATUS_PENDING
                _id:    new ObjectID()
