多对多关系<EntityFramework6.0>

无负载建立多对多关联的模型

原文中是Modeling a Many-to-Many Relationship with No Payload，虽然这么翻译也有点不准确，但是可以说明其目的，如下图所示，数据库中是这样的关系，

我们按照Database Frist的开发模式，由向导创建模型，会得到几个实体对象呢？

由此看出，这种多对多的关系宾补需要其它实体承载和转换，那么Modeling a Many-to-Many Relationship with No Payload也就这个意思咯。

再看看它是怎么工作的：

 　　　　　　using (var context = new EF6RecipesContext__1())
            {
                // add an artist with two albums
                var artist = new Artist { FirstName = "Alan", LastName = "Jackson" };
                var album1 = new Album { AlbumName = "Drive" };
                var album2 = new Album { AlbumName = "Live at Texas Stadium" };
                artist.Albums.Add(album1);
                artist.Albums.Add(album2);
                context.Artists.Add(artist);
                // add an album for two artists
                var artist1 = new Artist { FirstName = "Tobby", LastName = "Keith" };
                var artist2 = new Artist { FirstName = "Merle", LastName = "Haggard" };
                var album = new Album { AlbumName = "Honkytonk University" };
                artist1.Albums.Add(album);
                artist2.Albums.Add(album);
                context.Albums.Add(album);
                context.SaveChanges();
            }
            using (var context = new EF6RecipesContext__1())
            {
                Console.WriteLine("Artists and their albums...");
                var artists = context.Artists;
                foreach (var artist in artists)
                {
                    Console.WriteLine("{0} {1}", artist.FirstName, artist.LastName);
                    foreach (var album in artist.Albums)
                    {
                        Console.WriteLine("	{0}", album.AlbumName);
                    }
                }
                Console.WriteLine("
Albums and their artists...");
                var albums = context.Albums;
                foreach (var album in albums)
                {
                    Console.WriteLine("{0}", album.AlbumName);
                    foreach (var artist in album.Artists)
                    {
                        Console.WriteLine("	{0} {1}", artist.FirstName, artist.LastName);
                    }
                }
            }

建立多对多关联的模型并添加额外的信息

上面的LinkTable只包含了两个外键信息，EntityFramework并不会为此生成一个EntityType，但是如果LinkTable中附加了一些信息，EntityFramework就会为此生成一个EntityType，形成两个一对多的关联如下：

那么根据向导生成的实体关系图如下：

再看看它是如何让工作的【书中这段代码有点问题，现更正如下】

using (var context = new EF6_2RecipesContext())
            {
                var order = new Order
                {
                    OrderId = 1,
                    OrderDate = new DateTime(2010, 1, 18)
                };
                var item = new Item
                {
                    SKU = 1729,
                    Description = "Backpack",
                    Price = 29.97M
                };
                var oi = new OrderItem { Order = order, Item = item, Count = 1 };
                context.OrderItems.Add(oi);
                item = new Item
                {
                    SKU = 2929,
                    Description = "Water Filter",
                    Price = 13.97M
                };
                oi = new OrderItem { Order = order, Item = item, Count = 3 };
                context.OrderItems.Add(oi);
                item = new Item
                {
                    SKU = 1847,
                    Description = "Camp Stove",
                    Price = 43.99M
                };
                oi = new OrderItem { Order = order, Item = item, Count = 1 };
                context.OrderItems.Add(oi);
                context.SaveChanges();
            }
            using (var context = new EF6_2RecipesContext())
            {
                foreach (var order in context.Orders)
                {
                    Console.WriteLine("Order # {0}, ordered on {1}", order.OrderId,order.OrderDate);
                    Console.WriteLine("SKU	Description	Qty	Price");
                    Console.WriteLine("---	-----------	---	-----");
                    foreach (var oi in order.OrderItems)
                    {
                        Console.WriteLine("{0}	{1}	{2}	{3}", oi.Item.SKU,
                        oi.Item.Description, oi.Count,
                        oi.Item.Price.ToString("C"));
                    }
                }
            }

这也是书中推荐的建立多对多关系的方式，当然，项目的初期可能不需要添加额外的信息，而形成的第一种多对多的模式，那么书中也给出了最简单也是最优的解决方案，就是在LinkTable中额外使用一个Int类型的标识列即可，这样，到后期就可以体现出它的灵活性了。