public async Task CheckGettingTheReceiverAsync()
        {
            ApplicationDbContext db = GetDb();

            var userChatGroupsRepository = new EfDeletableEntityRepository <UserChatGroup>(db);
            var chatGroupsRepository     = new EfDeletableEntityRepository <ChatGroup>(db);

            var service = new ChatsService(
                this.chatMessagesRepository.Object,
                chatGroupsRepository,
                userChatGroupsRepository);

            var currentSender = new ApplicationUser()
            {
                Id       = "20",
                UserName = "******",
            };

            var currentReceiver = new ApplicationUser()
            {
                Id       = "21",
                UserName = "******",
            };

            await service.CreateUsersGroupAsync(currentSender, currentReceiver, "chat name");

            var groupId = await service.GetGroupIdAsync("chat name");

            var firstReceiverId = await service.GetReceiverAsync(groupId, currentSender.UserName);

            var secondReceiverId = await service.GetReceiverAsync(groupId, currentReceiver.UserName);

            Assert.Equal(currentReceiver.Id, firstReceiverId);
            Assert.Equal(currentSender.Id, secondReceiverId);
        }