private async void BeUniqueByExternalId_Create_Not_Exists()
        {
            Mock <IBucketRepository> bucketRepository = new Mock <IBucketRepository>();

            bucketRepository.Setup(x => x.ByExternalId(It.IsAny <Guid>())).Returns(Task.FromResult <Bucket>(null));
            var validator = new ApiBucketServerRequestModelValidator(bucketRepository.Object);

            await validator.ValidateCreateAsync(new ApiBucketServerRequestModel());

            validator.ShouldNotHaveValidationErrorFor(x => x.ExternalId, Guid.Parse("8420cdcf-d595-ef65-66e7-dff9f98764da"));
        }