public async Task <SimpleTask> CompleteAsync(CompleteUncomplete model)
        {
            if (!Exists(model.Id, model.UserId))
            {
                throw new ValidationException("Unauthorized");
            }

            ToDoTask originalTask = await _tasksRepository.CompleteAsync(model.Id, model.UserId);

            var result = _mapper.Map <SimpleTask>(originalTask);

            return(result);
        }