public async Task Should_ReturnNotFound_When_RetrievingNotExistingMailingGroupForUser(int mailingGroupId, int userId)
        {
            var cancellationToken = new CancellationToken();

            var databaseContext = Create.MockedDbContextFor <DatabaseContext>();

            databaseContext.Set <MailingGroup>().Add(new MailingGroup()
            {
                Id           = 1,
                SystemUserId = 1,
            });
            databaseContext.Set <MailingGroup>().Add(new MailingGroup()
            {
                Id           = 2,
                SystemUserId = 2,
            });

            databaseContext.SaveChanges();

            var request = new RetrieveSingleMailingGroupRequest(mailingGroupId);

            request.SetUserId(userId);

            var result = await new RetrieveSingleMailingGroupHandler(databaseContext).Handle(request, cancellationToken);

            Assert.Multiple(() =>
            {
                Assert.IsFalse(result.Success);
                Assert.AreEqual(result.StatusCode, HttpStatusCode.NotFound);
                Assert.IsNull(result.MailingGroup);
            });
        }