[Fact]
public void TestWhatHappensIfRelationshipLinkAndForeignKeyAreBothSetOk()
{
    //SETUP
    var sqlOptions = SqliteInMemory.CreateOptions<EfCoreContext>();
    using (var context = new EfCoreContext(sqlOptions))
    {
        context.Database.EnsureCreated();

        //ATTEMPT
        var book1 = new Book
        {
            Title = "Test1",
            Reviews = new List<Review>()
        };
        var book2 = new Book
        {
            Title = "Test2",
        };
        context.Add(book2);
        context.SaveChanges();

        //This review has its FK set to book2, but its added to book1
        var review = new Review { BookId = book2.BookId, NumStars = 1 };
        book1.Reviews.Add(review);
        context.Add(book1);
        context.SaveChanges();

        //VERIFY
        var books = context.Books.Include(x => x.Reviews).OrderBy(x => x.Title).ToList();
        //This shows that adding the review to book1's Reviews navigational collection wins over the FK being set.
        books.Select(x => x.Reviews.Count).ShouldEqual(new[] { 1, 0 });
    }
}

[Fact]
public void TestCopyOrderOk()
{
    //SETUP
    var sqlOptions = SqliteInMemory.CreateOptions<EfCoreContext>();
    using (var context = new EfCoreContext(sqlOptions))
    {
        context.Database.EnsureCreated();

        var books = context.SeedDatabaseFourBooks();                
        var order = new Order                                       
        {
            CustomerId = Guid.Empty,                                
            LineItems = new List<LineItem>
            {
                new LineItem                                        
                {                                                   
                    LineNum = 1, ChosenBook = books[0], NumBooks = 1
                },                                                  
                new LineItem                                        
                {                                                   
                    LineNum = 2, ChosenBook = books[1], NumBooks = 2
                },                                                  
            }
        };
        context.Add(order);                                         
        context.SaveChanges();                                      
    }
    using (var context = new EfCoreContext(sqlOptions))
    {
        //ATTEMPT
        var order = context.Orders                   
            .AsNoTracking()                          
            .Include(x => x.LineItems)               
            // no include for the Book entity because I don't want to copy the book entity.                                        
            .Single();                               

        order.OrderId = default;                     
        order.LineItems.First().LineItemId = default;
        order.LineItems.Last().LineItemId = default; 
        context.Add(order);                          
        context.SaveChanges();    

        //VERIFY part left out
}