/**
  * deletes the selected building
  *
  * @param $city_id
  * @param $slot_num
  */
 public function getDeleteBuilding($city_id, $slot_num, $building_id)
 {
     $city = City::find($city_id);
     if (!$this->validateOwner($city)) {
         return redirect('/home')->withErrors('Nem a te városod');
     }
     if ($building = $this->buildingCompleted($building_id)) {
         $city->human_resources->add(['workers' => $building->workers]);
         $slot = "slot{$slot_num}";
         $city->building_slot->{$slot} = 0;
         $city->building_slot->save();
         $building->task->each(function ($task) {
             TaskController::undoTask($task);
             $task->delete();
         });
         $building->delete();
         return redirect("/city/{$city_id}");
     }
     return redirect("/city/{$city_id}")->withErrors(['not_yet' => 'Az épület még nincs kész']);
 }