EF--payload or not

负载加载非负载加载适用于多对多场境。

一、非负载(payload-free)加载

　　1.1创建表

create table Album

(

    AlbumId int primary key identity(1,1),

    AlbumName varchar(20)

)

create table Artist

(

 ArtistId int identity(1,1) primary key,

 FirstName varchar(20),

 MiddleName varchar(20),

 LastName varchar(20)

)

create table LinkTable

(

    ArtistId int ,

    AlbumId int ,

    primary key (ArtistId, AlbumId),

    foreign key (ArtistId) references Artist(ArtistId),

    foreign key (AlbumId) references Album(AlbumId)

)

　　1.2、添加项目及数据实体对象

　　新建控制台项目，使用DBFirst添加生成Model对象，生成对象时选择添加的表。实体添加后如下

　　非负载(payload)加载时中间表LinkTable除了主键没有多余属性，EF不会将这类表生成实体而是做为实体关系生成。

　　1.3、数据操作

　　操作代码：　　

  class Program

    {

        static void Main(string[] args)

        {

            using (var context = new EF6RecipesEntities())

            {

                //Artist对应多个Album

                var artist = new Artist { FirstName = "Alan", LastName = "Jackson" };

                var album1 = new Album { AlbumName = "Drive" };

                var album2 = new Album { AlbumName = "Live at " };

                artist.Albums.Add(album1);

                artist.Albums.Add(album2);

                context.Artists.Add(artist);

                // Album对应多个Artist

                var artist1 = new Artist { FirstName = "Tobby", LastName = "Keith" };

                var artist2 = new Artist { FirstName = "Merle", LastName = "Haggard" };

                var album = new Album { AlbumName = "Honkytonk" };

                artist1.Albums.Add(album);

                artist2.Albums.Add(album);

                context.Albums.Add(album);

                context.SaveChanges();

                Console.WriteLine("Artists and their albums");

                var artists = context.Artists;

                foreach (var item in artists)

                {

                    Console.WriteLine("{0}{1}", item.FirstName, item.LastName);

                    foreach (var album_temp in item.Albums)

                    {

                        Console.WriteLine("\t{0}", album_temp.AlbumName);

                    }

                }

                Console.WriteLine("Artists and their albums");

                var Albums = context.Albums;

                foreach (var item_temp in Albums)

                {

                    Console.WriteLine("{0}", item_temp.AlbumName);

                    foreach (var art_temp in item_temp.Artists)

                    {

                        Console.WriteLine("\t{0}{1}", art_temp.FirstName, art_temp.LastName);

                    }

                }

                Console.ReadKey();

            }

        }

    }

　　运行结果如下:

二、负载(payload-rich)加载

　　2.1 修改表

　　继续使用上面的项目，删除重建LinkTable表，并清除旧数据。　　

--删除重建关联表

drop table linkTable

create table LinkTable

(

    ArtistId int ,

    AlbumId int ,

    comment varchar(200),

    primary key (ArtistId, AlbumId),

    foreign key (ArtistId) references Artist(ArtistId),

    foreign key (AlbumId) references Album(AlbumId)

)

--清空旧数据

delete from Album

delete from Artist

　　2.2重新生成Model实体

　　在项目打开实体视图，删除所有已添加的对象。再右键选择从数据库更新模型。选择表生成后实体关系如下。

　　由于中间表有标量属性comment，所以EF在生成实体时会连接表做为实体生成，并且生成两个一对多关系。

　　2.3数据操作

    class Program

    {

        static void Main(string[] args)

        {

            using (var context = new EF6RecipesEntities())

            {

                //Artist对应多个Album

                var artist = new Artist { FirstName = "Alan", LastName = "Jackson" };

                var album1 = new Album { AlbumName = "Drive" };

                var linkTab = new LinkTable { Album = album1, Artist = artist, comment = "Comment1" };

                var album2 = new Album { AlbumName = "Drive album2" };

                linkTab = new LinkTable { Artist = artist, Album = album2, comment = "Comment2" };

                context.Artists.Add(artist);

                context.SaveChanges();

                context.LinkTables.Add(linkTab);

                context.SaveChanges();

                foreach (var item in context.Artists)

                {

                    Console.WriteLine("{0}{1}", item.FirstName, item.LastName);

                    foreach (var table in item.LinkTables)

                    {

                        Console.WriteLine("\t{0}{1}", table.comment, table.Album.AlbumName);

                    }

                }

                Console.ReadKey();

            }

        }

    }

　　2.4运行结果

　　2.5 最佳实践

　　负载(payload-rich)情形下，为了适应扩展和对应关系，这种情况下中间表设置自己的主键。

巴特西

EF--payload or not

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