多对多关系<EntityFramework6.0>

2019-11-14 16:26:59

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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("/t{0}", album.AlbumName);                    }                }                Console.WriteLine("/nAlbums 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("/t{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/tDescription/tQty/tPrice");                    Console.WriteLine("---/t-----------/t---/t-----");                    foreach (var oi in order.OrderItems)                    {                        Console.WriteLine("{0}/t{1}/t{2}/t{3}", oi.Item.SKU,                        oi.Item.Description, oi.Count,                        oi.Item.Price.ToString("C"));                    }                }            }