public void AddItem_GivenANewItem_AddsToDatabase()
{
// arrange
OrderItem item = new OrderItem()
{
ID = 23
};
Mock <DbSet <OrderItem> > mockSet = new Mock <DbSet <OrderItem> >();
Mock <OrderDb> mockContext = new Mock <OrderDb>();
mockContext.Setup(c => c.OrderItems).Returns(mockSet.Object);
OrderItemRepository sut = new OrderItemRepository(mockContext.Object);
// act
sut.AddItem(item);
// assert
mockSet.Verify(s => s.Add(It.Is <OrderItem>(oi => oi.ID == 23)), Times.Once());
mockContext.Verify(c => c.SaveChanges(), Times.Once());
}