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 = ,
OrderDate = new DateTime(, , )
};
var item = new Item
{
SKU = ,
Description = "Backpack",
Price = 29.97M
};
var oi = new OrderItem { Order = order, Item = item, Count = };
context.OrderItems.Add(oi);
item = new Item
{
SKU = ,
Description = "Water Filter",
Price = 13.97M
};
oi = new OrderItem { Order = order, Item = item, Count = };
context.OrderItems.Add(oi);
item = new Item
{
SKU = ,
Description = "Camp Stove",
Price = 43.99M
};
oi = new OrderItem { Order = order, Item = item, Count = };
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"));
}
}
}
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。
