实体间的关系，简单来说无非就是一对一、一对多、多对多，根据方向性来说又分为双向和单向。Code First在实体关系上有以下约定：

1. 两个实体，如果一个实体包含一个引用属性，另一个实体包含一个集合属性，Code First默认约定它们为一对多关系。 
2. 两个实体，如果只有一个实体包含一个导航属性或一个集合属性，Code First也默认约定它们是一对多关系。 
3. 两个实体分别包含一个集合属性，Code First默认约定它们为多对多关系。 
4. 两个实体分别包含一个引用属性，Code First默认约定它们为一对一关系。 
5. 在一对一关系情况下，需要提供给Code First额外的信息，以确定它们的主从关系。 
6. 在实体中定义一个外键属性，Code First使用属性是否为空来确定关系是必须还是可选。

一、一对一

在Code First中，一对一关系总是需要配置，因为两个实体都包含有一个引用属性，无法确定它们的主从关系。

配置一对一关系常用的方法：

HasRequired ,HasOptional ,WithOptional ,WithRequiredPrincipal,WithRequiredDependent

下面是用到的类：

  1.    1:      public class Person
  1.    2:      {
  1.    3:          public int PersonId { get; set; }
  1.    4:          public int SocialSecurityNumber { get; set; }
  1.    5:          public string FirstName { get; set; }
  1.    6:          public string LastName { get; set; }
  1.    7:          public byte[] RowVersion { get; set; }
  1.    8:          public PersonPhoto Photo { get; set; }
  1.    9:      }
  1.   10:   
  1.   11:      public class PersonPhoto
  1.   12:      {
  1.   13:          public int PersonId { get; set; }
  1.   14:          public byte[] Photo { get; set; }
  1.   15:          public string Caption { get; set; }
  1.   16:          public Person PhotoOf { get; set; }
  1.   17:      }

因为Photo是具体人的，所以PersonPhoto使用PersonId作为主键。

下面是一对一关系配置的几种情况：

1.PersonPhoto必须属于一个Person，但是Person不一定有PersonPhoto，这种关系是1:0..1，此种情况下Person是一定存在的，所以它是主从关系主的一方。

  1.    1:  HasRequired(t => t.PhotoOf).WithOptional(t => t.Photo);

或

  1.    1:  HasOptional(t => t.Photo).WithRequired(t => t.PhotoOf);

2.PersonPhoto必须属于一个Person，Person也必须有PersonPhoto，这种关系式1:1，此种情况下，两个都一定存在，要确定主从关系，需要使用WithRequiredPrincipal或WithRequiredDependent。

  1.    1:  HasRequired(t => t.PhotoOf).WithRequiredDependent(t => t.Photo);

或

  1.    1:  HasRequired(t => t.Photo).WithRequiredPrincipal(t => t.PhotoOf);

上述两种情况都是真实存在的，不真实存在的就不说了。

下面配置一对一关系贴出Demo：

  1.    1: public class Person
  1.    2: {
  1.    3:     public int PersonId { get; set; }
  1.    4:     public int SocialSecurityNumber { get; set; }
  1.    5:     public string FirstName { get; set; }
  1.    6:     public string LastName { get; set; }
  1.    7:     public byte[] RowVersion { get; set; }
  1.    8:     public PersonPhoto Photo { get; set; }
  1.    9: }
  1.   10:  
  1.   11: public class PersonPhoto
  1.   12: {
  1.   13:     public int PersonId { get; set; }
  1.   14:     public byte[] Photo { get; set; }
  1.   15:     public string Caption { get; set; }
  1.   16:     public Person PhotoOf { get; set; }
  1.   17: }
  1.   18:  
  1.   19: //配置Person
  1.   20: public class PersonConfiguration : EntityTypeConfiguration<Person>
  1.   21: {
  1.   22:     public PersonConfiguration()
  1.   23:     {
  1.   24:         //主键
  1.   25:         HasKey(t => t.PersonId);
  1.   26:         //并发检查
  1.   27:         Property(t => t.SocialSecurityNumber).HasDatabaseGeneratedOption(DatabaseGeneratedOption.None).IsConcurrencyToken();
  1.   28:         //长度50 不为空
  1.   29:         Property(t => t.FirstName).IsRequired().HasMaxLength(50);
  1.   30:         //长度50 不为空
  1.   31:         Property(t => t.LastName).IsRequired().HasMaxLength(50);
  1.   32:         //并发检查
  1.   33:         Property(t => t.RowVersion).IsRowVersion();
  1.   34:         //HasRequired(t => t.Photo).WithRequiredPrincipal(t => t.PhotoOf);
  1.   35:         //HasOptional(t => t.Photo).WithRequired(t => t.PhotoOf);
  1.   36:     }
  1.   37: }
  1.   38:  
  1.   39: //配置PersonPhoto
  1.   40: public class PersonPhotoConfiguration : EntityTypeConfiguration<PersonPhoto>
  1.   41: {
  1.   42:     public PersonPhotoConfiguration()
  1.   43:     {
  1.   44:         //主键
  1.   45:         HasKey(t => t.PersonId);
  1.   46:         //长度50
  1.   47:         Property(t => t.Caption).HasMaxLength(50);
  1.   48:         //必须从属于Person
  1.   49:         HasRequired(t => t.PhotoOf).WithRequiredDependent(t => t.Photo);
  1.   50:     }
  1.   51: }
  1.   52:  
  1.   53: public class BreakAwayContext : DbContext
  1.   54: {
  1.   55:     public DbSet<Person> People { get; set; }
  1.   56:     public DbSet<PersonPhoto> Photos { get; set; }
  1.   57:  
  1.   58:     protected override void OnModelCreating(DbModelBuilder modelBuilder)
  1.   59:     {
  1.   60:         modelBuilder.Configurations.Add(new PersonConfiguration());
  1.   61:         modelBuilder.Configurations.Add(new PersonPhotoConfiguration());
  1.   62:         base.OnModelCreating(modelBuilder);
  1.   63:     }
  1.   64: }
  1.   65:  
  1.   66: public class Initializer : DropCreateDatabaseAlways<BreakAwayContext>
  1.   67: {
  1.   68:     public Initializer()
  1.   69:     {
  1.   70:     }
  1.   71:  
  1.   72:     //创建数据库时 Seed数据
  1.   73:     protected override void Seed(BreakAwayContext context)
  1.   74:     {
  1.   75:         context.People.Add(new Person()
  1.   76:         {
  1.   77:             FirstName = "E",
  1.   78:             LastName = "F",
  1.   79:             SocialSecurityNumber = 123456,
  1.   80:             Photo = new PersonPhoto()
  1.   81:             {
  1.   82:                 Caption = "这是照片",
  1.   83:                 Photo = new byte[] { }
  1.   84:             }
  1.   85:         });
  1.   86:         context.SaveChanges();
  1.   87:     }
  1.   88: }

测试程序

  1.    1: [TestClass]
  1.    2: public class UnitTest1
  1.    3: {
  1.    4:     [TestMethod]
  1.    5:     public void ShouldReturnAPersonWithPhoto()
  1.    6:     {
  1.    7:         //Arrange
  1.    8:         var init = new Initializer();
  1.    9:         Person person;
  1.   10:         using (var context = new BreakAwayContext())
  1.   11:         {
  1.   12:             init.InitializeDatabase(context);
  1.   13:             //Act
  1.   14:             person = context.People.Include(t => t.Photo).FirstOrDefault();
  1.   15:         }
  1.   16:         //Assert
  1.   17:         Assert.IsNotNull(person);
  1.   18:         Assert.IsNotNull(person.Photo);
  1.   19:     }
  1.   20: }

测试结果：

二、一对多

下面是用到的类：

  1.    1:      public class Blog
  1.    2:      {
  1.    3:          public Blog()
  1.    4:          {
  1.    5:              Posts = new List<Post>();
  1.    6:          }
  1.    7:   
  1.    8:          public int Id { get; set; }
  1.    9:          public DateTime Creationdate { get; set; }
  1.   10:          public string ShortDescription { get; set; }
  1.   11:          public string Title { get; set; }
  1.   12:          public List<Post> Posts { get; set; }
  1.   13:      }
  1.   14:   
  1.   15:      public class Post
  1.   16:      {
  1.   17:          public int Id { get; set; }
  1.   18:          public string Title { get; set; }
  1.   19:          public string Content { get; set; }
  1.   20:          public DateTime PostedDate { get; set; }
  1.   21:   
  1.   22:          public Nullable<int> BlogId { get; set; }
  1.   23:          public virtual Blog Blog { get; set; }
  1.   24:   
  1.   25:          public int PrimaryAuthorId { get; set; }
  1.   26:          public virtual Author PrimaryAuthor { get; set; }
  1.   27:          public Nullable<int> SecondaryAuthorId { get; set; }
  1.   28:          public virtual Author SecondaryAuthor { get; set; }
  1.   29:      }
  1.   30:   
  1.   31:      public class Author
  1.   32:      {
  1.   33:          public int Id { get; set; }
  1.   34:          public string Name { get; set; }
  1.   35:          public string Email { get; set; }
  1.   36:          //个人简历
  1.   37:          public string Bio { get; set; }
  1.   38:   
  1.   39:          public List<Post> PrimaryAuthorFor { get; set; }
  1.   40:          public List<Post> SecondaryAuthorFor { get; set; }
  1.   41:      }

配置一对多关系常用的方法有：

HasOptional ,HasRequired ,HasMany

Has方法后面往往跟着With方法

WithOptional ,WithRequired ,WithMany

下面配置一对多的几种情况：

1.Post一定归属于一个Blog，这种关系是1:n。

  1.    1:  HasMany(x => x.Posts).WithRequired(x =>x.Blog)

或

  1.    1:  HasRequired(x => x.Blog).WithMany(x => x.Posts)

2.Post可以单独存在，不用归属于Blog，这种关系是0..1:n。

  1.    1:  HasMany(x => x.Posts).WithOptional(x => x.Blog)

或

  1.    1:  HasOptional(x => x.Blog).WithMany(x => x.Posts)

设置外键

外键的默认约定：

[Target Type Key Name], [Target Type Name] + [Target Type Key Name], or [Navigation 
Property Name] + [Target Type Key Name]

本例中，匹配的是[Target Type Name] + [Target Type Key Name]，目标类型是Blog，目标类型主键是Id，加起来就是BlogId。下面使用Fluent API显示设置外键：

  1.    1:  HasMany(x => x.Posts).WithOptional(x => x.Blog).HasForeignKey(x => x.BlogId)

设置级联删除

  1.    1:  HasMany(x => x.Posts).WithOptional(x => x.Blog).HasForeignKey(x => x.BlogId).WillCascadeOnDelete();

反转属性

在Post实体中，有两个属性：PrimaryAuthor和SecondaryAuthor，第一作者和第二作者。在Author中有两个集合属性，Code First默认不能确定哪个集合属性和Post中的导航属性相匹配。使用Fluent API配置反转属性，如下：

  1.    1:  HasRequired(t => t.PrimaryAuthor).WithMany(t => t.PrimaryAuthorFor);
  1.    2:  HasOptional(t => t.SecondaryAuthor).WithMany(t => t.SecondaryAuthorFor);

下面是配置一对多关系的Demo:

  1.    1: public class Blog
  1.    2: {
  1.    3:     public Blog()
  1.    4:     {
  1.    5:         Posts = new List<Post>();
  1.    6:     }
  1.    7:  
  1.    8:     public int Id { get; set; }
  1.    9:     public DateTime Creationdate { get; set; }
  1.   10:     public string ShortDescription { get; set; }
  1.   11:     public string Title { get; set; }
  1.   12:     public List<Post> Posts { get; set; }
  1.   13: }
  1.   14:  
  1.   15: public class Post
  1.   16: {
  1.   17:     public int Id { get; set; }
  1.   18:     public string Title { get; set; }
  1.   19:     public string Content { get; set; }
  1.   20:     public DateTime PostedDate { get; set; }
  1.   21:  
  1.   22:     //Post可以不归属到Blog独立存在，注意这里的外键属性要设置为可空的
  1.   23:     public Nullable<int> BlogId { get; set; }
  1.   24:     public virtual Blog Blog { get; set; }
  1.   25:  
  1.   26:     public int PrimaryAuthorId { get; set; }
  1.   27:     public virtual Author PrimaryAuthor { get; set; }
  1.   28:     public Nullable<int> SecondaryAuthorId { get; set; }
  1.   29:     public virtual Author SecondaryAuthor { get; set; }
  1.   30: }
  1.   31:  
  1.   32: public class Author
  1.   33: {
  1.   34:     public int Id { get; set; }
  1.   35:     public string Name { get; set; }
  1.   36:     public string Email { get; set; }
  1.   37:     //个人简历
  1.   38:     public string Bio { get; set; }
  1.   39:  
  1.   40:     public List<Post> PrimaryAuthorFor { get; set; }
  1.   41:     public List<Post> SecondaryAuthorFor { get; set; }
  1.   42: }
  1.   43:  
  1.   44: public class BlogConfiguratioin : EntityTypeConfiguration<Blog>
  1.   45: {
  1.   46:     public BlogConfiguratioin()
  1.   47:     {
  1.   48:         ToTable("Blogs");
  1.   49:         HasKey(t => t.Id);
  1.   50:         Property(t => t.Id).HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);
  1.   51:         Property(t => t.Title).IsRequired().HasMaxLength(250);
  1.   52:         Property(t => t.Creationdate).HasColumnName("CreationDate").IsRequired();
  1.   53:         Property(t => t.ShortDescription).HasColumnType("Text").IsMaxLength().IsOptional().HasColumnName("Description");
  1.   54:         //配置Blog和Post的一对多关系，Blog对Post是可选的，外键BlogId，并设置为级联删除
  1.   55:         HasMany(t => t.Posts).WithOptional(t => t.Blog).HasForeignKey(t => t.BlogId).WillCascadeOnDelete();
  1.   56:     }
  1.   57: }
  1.   58:  
  1.   59: public class PostConfiguration : EntityTypeConfiguration<Post>
  1.   60: {
  1.   61:     public PostConfiguration()
  1.   62:     {
  1.   63:         ToTable("Posts");
  1.   64:         HasKey(t => t.Id);
  1.   65:         Property(t => t.Id).HasColumnName("PostId").HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);
  1.   66:         Property(t => t.Content).HasColumnName("Body").IsMaxLength();
  1.   67:         Property(t => t.PostedDate).HasColumnName("PostedDate");
  1.   68:         Property(t => t.Title).HasColumnName("Title").IsMaxLength();
  1.   69:         //配置反转属性，集合属性PrimaryAuthorFor匹配PrimaryAuthor
  1.   70:         HasRequired(t => t.PrimaryAuthor).WithMany(t => t.PrimaryAuthorFor);
  1.   71:         //配置反转属性,集合属性SecondaryAuthorFor匹配SecondaryAuthor
  1.   72:         HasOptional(t => t.SecondaryAuthor).WithMany(t => t.SecondaryAuthorFor);
  1.   73:     }
  1.   74: }
  1.   75:  
  1.   76: public class AuthorConfiguration : EntityTypeConfiguration<Author>
  1.   77: {
  1.   78:     public AuthorConfiguration()
  1.   79:     {
  1.   80:         ToTable("Authors");
  1.   81:         HasKey(t => t.Id).Property(t => t.Id).HasColumnName("AuthorId").HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);
  1.   82:         Property(t => t.Name).IsRequired().HasMaxLength(50);
  1.   83:         Property(t => t.Email).IsRequired().HasMaxLength(50);
  1.   84:         Property(t => t.Bio).HasMaxLength(1000);
  1.   85:     }
  1.   86: }
  1.   87:  
  1.   88: public class BreakAwayContext : DbContext
  1.   89: {
  1.   90:     public DbSet<Blog> Blogs { get; set; }
  1.   91:     public DbSet<Post> Posts { get; set; }
  1.   92:     public DbSet<Author> Authors { get; set; }
  1.   93:  
  1.   94:     protected override void OnModelCreating(DbModelBuilder modelBuilder)
  1.   95:     {
  1.   96:         modelBuilder.Configurations.Add(new BlogConfiguratioin());
  1.   97:         modelBuilder.Configurations.Add(new PostConfiguration());
  1.   98:         modelBuilder.Configurations.Add(new AuthorConfiguration());
  1.   99:         base.OnModelCreating(modelBuilder);
  1.  100:     }
  1.  101: }
  1.  102:  
  1.  103: public class Initializer : DropCreateDatabaseAlways<BreakAwayContext>
  1.  104: {
  1.  105:     public Initializer()
  1.  106:     {
  1.  107:     }
  1.  108:  
  1.  109:     protected override void Seed(BreakAwayContext context)
  1.  110:     {
  1.  111:         var primaryAuthor = new Author()
  1.  112:         {
  1.  113:             Name = "张三",
  1.  114:             Email = "zhangsan@126.com",
  1.  115:             Bio = "张三的简历"
  1.  116:         };
  1.  117:         var secondaryAuthor = new Author()
  1.  118:         {
  1.  119:             Name = "李四",
  1.  120:             Email = "lisi@126.com",
  1.  121:             Bio = "李四的简历"
  1.  122:         };
  1.  123:         var blog = new Blog()
  1.  124:         {
  1.  125:             Title = "EF",
  1.  126:             ShortDescription = "关于EF的博客",
  1.  127:             Creationdate = DateTime.Now
  1.  128:         };
  1.  129:         blog.Posts.Add(new Post()
  1.  130:         {
  1.  131:             Title = "配置关系",
  1.  132:             PostedDate = DateTime.Now,
  1.  133:             Content = "这是Post的内容",
  1.  134:             PrimaryAuthor = primaryAuthor,
  1.  135:             SecondaryAuthor = secondaryAuthor
  1.  136:         });
  1.  137:         context.Blogs.Add(blog);
  1.  138:         context.SaveChanges();
  1.  139:     }
  1.  140: }

测试程序：

  1.    1: [TestClass]
  1.    2: public class OneToManyTest
  1.    3: {
  1.    4:     [TestMethod]
  1.    5:     public void ShouldReturnBlogWithPosts()
  1.    6:     {
  1.    7:         //Arrage
  1.    8:         Database.SetInitializer(new Initializer());
  1.    9:         var context = new BreakAwayContext();
  1.   10:         //Act
  1.   11:         var blog = context.Blogs.Include(t => t.Posts).FirstOrDefault();
  1.   12:         //Assert
  1.   13:         Assert.IsNotNull(blog);
  1.   14:         Assert.IsNotNull(blog.Posts);
  1.   15:         Assert.IsNotNull(blog.Posts.FirstOrDefault().PrimaryAuthor);
  1.   16:     }
  1.   17: }

测试结果：

三、多对多

下面是配置多对多关系用到的类，跟一对多差不多，只不过Post和Author的关系变成多对多的了。

  1.    1:      public class Post
  1.    2:      {
  1.    3:          public int Id { get; set; }
  1.    4:          public string Title { get; set; }
  1.    5:          public string Content { get; set; }
  1.    6:          public DateTime PostedDate { get; set; }
  1.    7:   
  1.    8:          public virtual List<Author> Authors { get; set; }
  1.    9:      }
  1.   10:   
  1.   11:      public class Author
  1.   12:      {
  1.   13:          public int Id { get; set; }
  1.   14:          public string Name { get; set; }
  1.   15:          public string Email { get; set; }
  1.   16:          //个人简历
  1.   17:          public string Bio { get; set; }
  1.   18:   
  1.   19:          public virtual List<Post> Posts { get; set; }
  1.   20:      }

一篇文章有多个作者，一个作者著有多篇文章。

配置多对多关系使用HasMany和WithMany方法，可以使用Map配置生成关联表的名字。

下面是配置多对多关系的Demo：

  1.    1: public class Post
  1.    2: {
  1.    3:     public Post()
  1.    4:     {
  1.    5:         Authors = new List<Author>();
  1.    6:     }
  1.    7:  
  1.    8:     public int Id { get; set; }
  1.    9:     public string Title { get; set; }
  1.   10:     public string Content { get; set; }
  1.   11:     public DateTime PostedDate { get; set; }
  1.   12:  
  1.   13:     public virtual List<Author> Authors { get; set; }
  1.   14: }
  1.   15:  
  1.   16: public class Author
  1.   17: {
  1.   18:     public Author()
  1.   19:     {
  1.   20:         Posts = new List<Post>();
  1.   21:     }
  1.   22:  
  1.   23:     public int Id { get; set; }
  1.   24:     public string Name { get; set; }
  1.   25:     public string Email { get; set; }
  1.   26:     //个人简历
  1.   27:     public string Bio { get; set; }
  1.   28:  
  1.   29:     public virtual List<Post> Posts { get; set; }
  1.   30: }
  1.   31:  
  1.   32: public class PostConfiguration : EntityTypeConfiguration<Post>
  1.   33: {
  1.   34:     public PostConfiguration()
  1.   35:     {
  1.   36:         ToTable("Posts");
  1.   37:         HasKey(t => t.Id);
  1.   38:         Property(t => t.Id).HasColumnName("PostId").HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);
  1.   39:         Property(t => t.Content).HasColumnName("Body").IsMaxLength();
  1.   40:         Property(t => t.PostedDate).HasColumnName("PostedDate");
  1.   41:         Property(t => t.Title).HasColumnName("Title").IsMaxLength();
  1.   42:         //配置多对多关系 ToTable 配置生成的关联表名字 MapLeftKey默认表示调用HasMany的实体的主键
  1.   43:         //本例中如果不使用MapLeftKey默认生成Post_Id
  1.   44:         HasMany(t => t.Authors).WithMany(t => t.Posts).Map(m =>
  1.   45:             {
  1.   46:                 m.ToTable("PostAuthor");
  1.   47:                 m.MapLeftKey("PostId");
  1.   48:                 m.MapRightKey("AuthorId");
  1.   49:             });
  1.   50:     }
  1.   51: }
  1.   52:  
  1.   53: public class AuthorConfiguration : EntityTypeConfiguration<Author>
  1.   54: {
  1.   55:     public AuthorConfiguration()
  1.   56:     {
  1.   57:         ToTable("Authors");
  1.   58:         HasKey(t => t.Id);
  1.   59:         Property(t => t.Id).HasColumnName("AuthorId").HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);
  1.   60:         Property(t => t.Bio).HasColumnType("Text").IsMaxLength();
  1.   61:         Property(t => t.Email).HasMaxLength(100).IsRequired();
  1.   62:         Property(t => t.Name).HasMaxLength(100).IsRequired();
  1.   63:     }
  1.   64: }
  1.   65:  
  1.   66: public class TestContext : DbContext
  1.   67: {
  1.   68:     public DbSet<Post> Posts { get; set; }
  1.   69:     public DbSet<Author> Authors { get; set; }
  1.   70:  
  1.   71:     protected override void OnModelCreating(DbModelBuilder modelBuilder)
  1.   72:     {
  1.   73:         modelBuilder.Configurations.Add(new PostConfiguration());
  1.   74:         modelBuilder.Configurations.Add(new AuthorConfiguration());
  1.   75:         base.OnModelCreating(modelBuilder);
  1.   76:     }
  1.   77: }
  1.   78:  
  1.   79: public class Initializer : DropCreateDatabaseAlways<TestContext>
  1.   80: {
  1.   81:     protected override void Seed(TestContext context)
  1.   82:     {
  1.   83:         var post = new Post()
  1.   84:         {
  1.   85:             Title = "Post1",
  1.   86:             Content = "Content1",
  1.   87:             PostedDate = DateTime.Now
  1.   88:         };
  1.   89:         var author = new Author()
  1.   90:         {
  1.   91:             Name = "张三",
  1.   92:             Email = "zhangsan@126.com",
  1.   93:             Bio = "张三的简历"
  1.   94:         };
  1.   95:         var author1 = new Author()
  1.   96:         {
  1.   97:             Name = "李四",
  1.   98:             Email = "lisi@126.com",
  1.   99:             Bio = "李四的简历"
  1.  100:         };
  1.  101:         var author2 = new Author()
  1.  102:         {
  1.  103:             Name = "王五",
  1.  104:             Email = "wangwu@126.com",
  1.  105:             Bio = "王五的简历"
  1.  106:         };
  1.  107:         post.Authors.Add(author);
  1.  108:         post.Authors.Add(author1);
  1.  109:         context.Posts.Add(post);
  1.  110:         post = new Post()
  1.  111:         {
  1.  112:             Title = "Post2",
  1.  113:             Content = "Content2",
  1.  114:             PostedDate = DateTime.Now
  1.  115:         };
  1.  116:         post.Authors.Add(author);
  1.  117:         post.Authors.Add(author2);
  1.  118:         context.Posts.Add(post);
  1.  119:         context.SaveChanges();
  1.  120:     }
  1.  121: }

测试程序：

  1.    1: [TestClass]
  1.    2: public class ManyToManyTest
  1.    3: {
  1.    4:     [TestMethod]
  1.    5:     public void ShouldReturnPostWithAuthors()
  1.    6:     {
  1.    7:         //Arrage
  1.    8:         var init = new Initializer();
  1.    9:         var context = new ManyToMany.TestContext();
  1.   10:         init.InitializeDatabase(context);
  1.   11:         //Act
  1.   12:         var post = context.Posts.Include(t => t.Authors).FirstOrDefault();
  1.   13:         //Assert
  1.   14:         Assert.IsNotNull(post);
  1.   15:         Assert.AreEqual(2, post.Authors.Count);
  1.   16:         Assert.AreEqual("李四", post.Authors[1].Name);
  1.   17:     }
  1.   18: }

测试结果：

现在关联表中只有两个字段，如下图所示：

如果再加个字段，比如DateAdd，这就需要给关联表定义一个实体。

  1.    1:      public class PostAuthor
  1.    2:      {
  1.    3:          public int PostId { get; set; }
  1.    4:          public int AuthorId { get; set; }
  1.    5:   
  1.    6:          public Post Post { get; set; }
  1.    7:          public Author Author { get; set; }
  1.    8:   
  1.    9:          public DateTime DateAdd { get; set; }
  1.   10:      }

另外需要在Post和Author实体中加入一个集合属性：

  1.    1:          public virtual List<PostAuthor> PostAuthors { get; set; }

另外还需要配置PostAuthor实体，具体代码如下面的Demo所示：

  1.    1: public class Post
  1.    2: {
  1.    3:     public Post()
  1.    4:     {
  1.    5:         PostAuthors = new List<PostAuthor>();
  1.    6:     }
  1.    7:  
  1.    8:     public int Id { get; set; }
  1.    9:     public string Title { get; set; }
  1.   10:     public string Content { get; set; }
  1.   11:     public DateTime PostedDate { get; set; }
  1.   12:  
  1.   13:     //public virtual List<Author> Authors { get; set; }
  1.   14:     public virtual List<PostAuthor> PostAuthors { get; set; }
  1.   15: }
  1.   16:  
  1.   17: public class Author
  1.   18: {
  1.   19:     public Author()
  1.   20:     {
  1.   21:         PostAuthors = new List<PostAuthor>();
  1.   22:     }
  1.   23:  
  1.   24:     public int Id { get; set; }
  1.   25:     public string Name { get; set; }
  1.   26:     public string Email { get; set; }
  1.   27:     //个人简历
  1.   28:     public string Bio { get; set; }
  1.   29:  
  1.   30:     //public virtual List<Post> Posts { get; set; }
  1.   31:     public virtual List<PostAuthor> PostAuthors { get; set; }
  1.   32: }
  1.   33:  
  1.   34: //关联表的实体
  1.   35: public class PostAuthor
  1.   36: {
  1.   37:     public int PostId { get; set; }
  1.   38:     public int AuthorId { get; set; }
  1.   39:  
  1.   40:     public Post Post { get; set; }
  1.   41:     public Author Author { get; set; }
  1.   42:  
  1.   43:     public DateTime? DateAdd { get; set; }
  1.   44: }
  1.   45:  
  1.   46: public class PostConfiguration : EntityTypeConfiguration<Post>
  1.   47: {
  1.   48:     public PostConfiguration()
  1.   49:     {
  1.   50:         ToTable("Posts");
  1.   51:         HasKey(t => t.Id);
  1.   52:         Property(t => t.Id).HasColumnName("PostId").HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);
  1.   53:         Property(t => t.Content).HasColumnName("Body").IsMaxLength();
  1.   54:         Property(t => t.PostedDate).HasColumnName("PostedDate");
  1.   55:         Property(t => t.Title).HasColumnName("Title").IsMaxLength();
  1.   56:     }
  1.   57: }
  1.   58:  
  1.   59: public class AuthorConfiguration : EntityTypeConfiguration<Author>
  1.   60: {
  1.   61:     public AuthorConfiguration()
  1.   62:     {
  1.   63:         ToTable("Authors");
  1.   64:         HasKey(t => t.Id);
  1.   65:         Property(t => t.Id).HasColumnName("AuthorId").HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);
  1.   66:         Property(t => t.Bio).HasColumnType("Text").IsMaxLength();
  1.   67:         Property(t => t.Email).HasMaxLength(100).IsRequired();
  1.   68:         Property(t => t.Name).HasMaxLength(100).IsRequired();
  1.   69:     }
  1.   70: }
  1.   71:  
  1.   72: //配置关联表实体
  1.   73: public class PostAuthorConfiguration : EntityTypeConfiguration<PostAuthor>
  1.   74: {
  1.   75:     public PostAuthorConfiguration()
  1.   76:     {
  1.   77:         ToTable("PostAuthors");
  1.   78:         //配置组合主键
  1.   79:         HasKey(t => new { t.PostId, t.AuthorId });
  1.   80:         Property(t => t.PostId).HasColumnOrder(0);
  1.   81:         Property(t => t.AuthorId).HasColumnOrder(1);
  1.   82:         //这里是配置一对多关系
  1.   83:         HasRequired(t => t.Post).WithMany(t => t.PostAuthors).HasForeignKey(t => t.PostId);
  1.   84:         HasRequired(t => t.Author).WithMany(t => t.PostAuthors).HasForeignKey(t => t.AuthorId);
  1.   85:     }
  1.   86: }
  1.   87:  
  1.   88: public class TestContext : DbContext
  1.   89: {
  1.   90:     public DbSet<Post> Posts { get; set; }
  1.   91:     public DbSet<Author> Authors { get; set; }
  1.   92:     public DbSet<PostAuthor> PostAuthors { get; set; }
  1.   93:  
  1.   94:     protected override void OnModelCreating(DbModelBuilder modelBuilder)
  1.   95:     {
  1.   96:         modelBuilder.Configurations.Add(new PostConfiguration());
  1.   97:         modelBuilder.Configurations.Add(new AuthorConfiguration());
  1.   98:         modelBuilder.Configurations.Add(new PostAuthorConfiguration());
  1.   99:         base.OnModelCreating(modelBuilder);
  1.  100:     }
  1.  101: }
  1.  102:  
  1.  103: public class Initializer : DropCreateDatabaseAlways<TestContext>
  1.  104: {
  1.  105:     protected override void Seed(TestContext context)
  1.  106:     {
  1.  107:         var post = new Post()
  1.  108:         {
  1.  109:             Title = "Post1",
  1.  110:             Content = "Content1",
  1.  111:             PostedDate = DateTime.Now
  1.  112:         };
  1.  113:         post = context.Posts.Add(post);
  1.  114:         var author = new Author()
  1.  115:         {
  1.  116:             Name = "张三",
  1.  117:             Email = "zhangsan@126.com",
  1.  118:             Bio = "张三的简历"
  1.  119:         };
  1.  120:         var author1 = new Author()
  1.  121:         {
  1.  122:             Name = "李四",
  1.  123:             Email = "lisi@126.com",
  1.  124:             Bio = "李四的简历"
  1.  125:         };
  1.  126:         author = context.Authors.Add(author);
  1.  127:         author1 = context.Authors.Add(author1);
  1.  128:         context.SaveChanges();
  1.  129:         PostAuthor pa1 = new PostAuthor()
  1.  130:         {
  1.  131:             PostId = post.Id,
  1.  132:             AuthorId = author.Id,
  1.  133:             DateAdd = DateTime.Now
  1.  134:         };
  1.  135:         PostAuthor pa2 = new PostAuthor()
  1.  136:         {
  1.  137:             PostId = post.Id,
  1.  138:             AuthorId = author1.Id,
  1.  139:             DateAdd = DateTime.Now
  1.  140:         };
  1.  141:         context.PostAuthors.Add(pa1);
  1.  142:         context.PostAuthors.Add(pa2);
  1.  143:         context.SaveChanges();
  1.  144:     }
  1.  145: }

测试程序：

  1.    1: [TestMethod]
  1.    2: public void ShouldReturnAuthorsWithDateAdd()
  1.    3: {
  1.    4:     //Arrage
  1.    5:     var init = new Initializer();
  1.    6:     var context = new ManyToMany.TestContext();
  1.    7:     init.InitializeDatabase(context);
  1.    8:     //Act
  1.    9:     var post = context.Posts.Include(t => t.PostAuthors).FirstOrDefault();
  1.   10:     //Assert
  1.   11:     Assert.IsNotNull(post);
  1.   12:     Assert.AreEqual(2, post.PostAuthors.Count);
  1.   13:     Assert.IsNotNull(post.PostAuthors[0].DateAdd);
  1.   14: }

测试结果：

生成的关联表如下图所示：

http://www.cnblogs.com/nianming/archive/2012/11/12/2767089.html

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