public void AddShipper_GivenANewShipper_AddsToDatabase()
        {
            // arrange
            Mock <DbSet <Shipper> > mockSet     = new Mock <DbSet <Shipper> >();
            Mock <ShippingDb>       mockContext = new Mock <ShippingDb>();

            mockContext.Setup(c => c.Shippers).Returns(mockSet.Object);
            ShipperRepository sut     = new ShipperRepository(mockContext.Object);
            Shipper           shipper = new Shipper()
            {
                ID = 200
            };

            // act
            sut.AddShipper(shipper);

            // assert
            mockSet.Verify(s => s.Add(It.Is <Shipper>(a => a.ID == 200)), Times.Once());
            mockContext.Verify(c => c.SaveChanges(), Times.Once());
        }