public void RequireUserIsNotChatMember(string username, Guid chatId)
        {
            var user = _userValidationService.RequireAndGetUserByUsername(username);

            var chatUser = _chatUserRepository.GetByUserIdAndChatId(user.Id, chatId);

            if (chatUser != null)
            {
                throw new LogicApiException("User is already member of this chat.");
            }
        }