无负载建立多对多关联的模型原文中是Modeling a Many-to-Many Relationshipwith 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(varcontext =newEF6RecipesContext__1())
{//add an artist with two albumsvarartist =newArtist { FirstName ="Alan", LastName ="Jackson"};varalbum1 =newAlbum { AlbumName ="Drive"};varalbum2 =newAlbum { AlbumName ="Live at Texas Stadium"};
artist.Albums.Add(album1);
artist.Albums.Add(album2);
context.Artists.Add(artist);//add an album for two artistsvarartist1 =newArtist { FirstName ="Tobby", LastName ="Keith"};varartist2 =newArtist { FirstName ="Merle", LastName ="Haggard"};varalbum =newAlbum { AlbumName ="Honkytonk University"};
artist1.Albums.Add(album);
artist2.Albums.Add(album);
context.Albums.Add(album);
context.SaveChanges();
}using(varcontext =newEF6RecipesContext__1())
{
Console.WriteLine("Artists and their albums...");varartists =context.Artists;foreach(varartistinartists)
{
Console.WriteLine("{0} {1}", artist.FirstName, artist.LastName);foreach(varalbuminartist.Albums)
{
Console.WriteLine("\t{0}", album.AlbumName);
}
}
Console.WriteLine("\nAlbums and their artists...");varalbums =context.Albums;foreach(varalbuminalbums)
{
Console.WriteLine("{0}", album.AlbumName);foreach(varartistinalbum.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(varcontext =newEF6_2RecipesContext())
{varorder =newOrder
{
OrderId=1,
OrderDate=newDateTime(2010,1,18)
};varitem =newItem
{
SKU=1729,
Description="Backpack",PRice=29.97M};varoi =newOrderItem { Order = order, Item = item, Count =1};
context.OrderItems.Add(oi);
item=newItem
{
SKU=2929,
Description="Water Filter",
Price=13.97M};
oi=newOrderItem { Order = order, Item = item, Count =3};
context.OrderItems.Add(oi);
item=newItem
{
SKU=1847,
Description="Camp Stove",
Price=43.99M};
oi=newOrderItem { Order = order, Item = item, Count =1};
context.OrderItems.Add(oi);
context.SaveChanges();
}using(varcontext =newEF6_2RecipesContext())
{foreach(varorderincontext.Orders)
{
Console.WriteLine("Order # {0}, ordered on {1}", order.OrderId,order.OrderDate);
Console.WriteLine("SKU\tDescription\tQty\tPrice");
Console.WriteLine("---\t-----------\t---\t-----");foreach(varoiinorder.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。