上一篇文章我们讲解了如何用 Fluent API 来配置/映射属性和类型，本文将把重点放在其是如何配置关系的。

　　文中所使用代码如下

public class Student    {        public int ID { get; set; }        public string Name { get; set; }        public DateTime EnrollmentDate { get; set; }        // Navigation properties        public virtual Address Address { get; set; }        public virtual OtherInfo OtherInfo { get; set; }        public virtual ICollection
       
         Enrollments { get; set; }    }    public class Department    {        public Department()        {            this.Courses = new HashSet
        
         ();        }        // Primary key        public int DepartmentID { get; set; }        public string Name { get; set; }        public decimal Budget { get; set; }        public System.DateTime StartDate { get; set; }        public int? Administrator { get; set; }        // Navigation property        public virtual ICollection
         
           Courses { get; private set; }    }    public class Course    {        public int CourseID { get; set; }        public string Title { get; set; }        public int Credits { get; set; }        // Foreign key        public int DepartmentID { get; set; }        public string DepartmentName { get; set; }        public int SomeDepartmentID { get; set; }        // Navigation properties        public virtual Department Department { get; set; }        public virtual ICollection
          
            Enrollments { get; set; }        public virtual ICollection
           
             Instructors { get; set; }    }    public class Instructor    {        public int InstructorID { get; set; }        public string Name { get; set; }        public DateTime HireDate { get; set; }        // Navigation properties        public virtual ICollection
            
              Courses { get; set; } } public class Enrollment { public int EnrollmentID { get; set; } public int CourseID { get; set; } public int StudentID { get; set; } // Navigation property public virtual Course Course { get; set; } public virtual Student Student { get; set; } } public class Address { public int AddressId { get; set; } public string HomeAddress { get; set; } public string LiveAddress { get; set; } // Navigation property public virtual Student Student { get; set; } } public class OtherInfo { public int Id { get; set; } public string HomeAddress { get; set; } public string MailAddress { get; set; } public string PhoneNumber { get; set; } public string StudentID { get; set; } // Navigation property public virtual Student Student { get; set; } }
            
           
          
         
        
       

 

EntityTypeConfiguration<TEntityType>

　　上面是泛型类的部分方法截图，一般我们通过 Code First Fluent API 来配置实体间的关系时都是从此泛型类的实例开始，此泛型类为我们提供了大部分的关系处理方法，如必须的 HasRequired ,可选的 HasOptional ,多个的 HasMany .

　　上面所有方法（除了 HasMany, HasOptional, HasRequired ）的返回值都是泛型类 EntityTypeConfiguration<TEntityType> 的实例本身，这给链式操作提供了极大地方便.

　　HasRequired, HasOptional, HasMany 这三个方法的参数都是一个 lambda expression, 只不过前两个用于表示实体关系（Relationship）间的导航属性（navigation property），后一个则是导航属性的集合（Collection）

　　HasRequired, HasOptional, HasMany 这三个方法的返回值都是可以继续进行配置的泛型类实例，虽然名称不同，方法名也略有不同，但方法主体所体现的思想是一致的

RequiredNavigationPropertyConfiguration<TEntityType, TTargetEntityType>

OptionalNavigationPropertyConfiguration<TEntityType, TTargetEntityType>

ManyNavigationPropertyConfiguration<TEntityType, TTargetEntityType>

　　三个泛型类都有类似于如下三个方法 WithRequired, WithOptional, WithMany, 这三个方法都提供了重载的无参版本，有参方法的参数也都是一个 lambda expression, 前两个用于表示实体关系（Relationship）间的导航属性（navigation property），后一个则是导航属性的集合（Collection）

　　接下来，你可以使用方法 HasForeignKey 继续配置外键属性，方法的参数仍然是一个 lambda expression, 只不过代表一个属性

DependentNavigationPropertyConfiguration<TDependentEntityType>

public CascadableNavigationPropertyConfiguration HasForeignKey
      
       (Expression
       
        
         > foreignKeyExpression);
        
       
      

　　其它两个类主要包含方法 Map ，如下

ForeignKeyNavigationPropertyConfiguration

public CascadableNavigationPropertyConfiguration Map(Action
      
        configurationAction);
      

ManyToManyNavigationPropertyConfiguration<TEntityType, TTargetEntityType>

public ManyToManyNavigationPropertyConfiguration
      
        Map(Action
       
         configurationAction);public ManyToManyNavigationPropertyConfiguration
        
          MapToStoredProcedures();public ManyToManyNavigationPropertyConfiguration
         
           MapToStoredProcedures(Action
          
           
            > modificationStoredProcedureMappingConfigurationAction);
           
          
         
        
       
      

　　我们可以继续在返回的实例上进行配置

CascadableNavigationPropertyConfiguration

public void WillCascadeOnDelete();public void WillCascadeOnDelete(bool value);

　　我们看到了级联删除

 

Configuring Relationships

1:1,0 - Configuring a Required-to-Optional Relationship (One-to–Zero-or-One)

　　一个学生可以有一个或没有其它信息（包括邮件地址、手机号等）

// Map one-to-zero or one relationshipmodelBuilder.Entity
      
       ()    .HasRequired(t => t.Student)    .WithOptional(t => t.OtherInfo);
      

1:1 - Configuring a Relationship Where Both Ends Are Required (One-to-One)

　　一个学生肯定有一个地址信息（包含家庭住址、居住地址）

// Map one-to-one relationshipmodelBuilder.Entity
()    .HasRequired(t => t.Student)    .WithRequiredPrincipal(t => t.Address);

1:N - Configuring a Required-to-Many Relationship (One-to-Many)

　　一个学生可以参加多门课程

// Map one-to-many relationshipmodelBuilder.Entity
      
       ()    .HasMany(t => t.Enrollments)    .WithRequired(t => t.Student);
      

N:N - Configuring a Many-to-Many Relationship (Many-to-Many)

　　一个老师可以教授多门课程，一门课程也可以由多名老师教授

// Map one-to-many relationshipmodelBuilder.Entity
      
       ()    .HasMany(t => t.Instructors)    .WithMany(t => t.Courses);
      

　　你还可以进一步指定中间连接表（数据库将会创建中间连接表）

// Map one-to-many relationshipmodelBuilder.Entity
      
       ()    .HasMany(t => t.Instructors)    .WithMany(t => t.Courses)    .Map(m =>    {        m.ToTable("CourseInstructor");        m.MapLeftKey("CourseID");        m.MapRightKey("InstructorID");    });
      

Configuring a Relationship with One Navigation Property - 基于导航属性配置关系

　　如果关系是单向的，即两个实体间只在一个实体上定义导航属性。Code First Conventions 能够自动推断这种关系为 one-to-many .

　　例如你想在 Student 和 Address 两个实体间建立 one-to-one 关系，而且只在 Address 实体上包含导航属性，此时你就需要用 Code First Fluent API 配置这种关系

// Map one-to-one relationshipmodelBuilder.Entity
()    .HasRequired(t => t.Student)    .WithRequiredPrincipal();

WillCascadeOnDelete - Enabling Cascade Delete (级联删除)

　　你可以使用 WillCascadeOnDelete 来级联删除关系，如果从属主体上的外键是 not nullable, 那么 Code First 将设置级联删除，否则将不会设置级联删除，而只是仅仅把外键设置成 null

 　　在 Code First Conventions 下是这样移除级联删除的

modelBuilder.Conventions.Remove
      
       ()modelBuilder.Conventions.Remove
       
        ()
       
      

　　Code First Fluent API 的方式如下

// Cascade DeletemodelBuilder.Entity
      
       ()    .HasRequired(t => t.Department)    .WithMany(t => t.Courses)    .HasForeignKey(d => d.DepartmentID)    .WillCascadeOnDelete(false);
      

Configuring a Composite Foreign Key - 配置组合外键

　　如果设置 Department 的主键为组合主键 DepartmentID, Name，则可以通过 Fluent API 为 Course 指定组合外键

// Composite primary keymodelBuilder.Entity
      
       ()    .HasKey(d => new { d.DepartmentID, d.Name });// Composite foreign keymodelBuilder.Entity
       
        ()    .HasRequired(c => c.Department)    .WithMany(d => d.Courses)    .HasForeignKey(d => new { d.DepartmentID, d.DepartmentName });
       
      

Renaming a Foreign Key That Is Not Defined in the Model - 重命名外键

　　可以重命名外键名

// Renaming a Foreign Key That Is Not Defined in the ModelmodelBuilder.Entity
      
       ()    .HasRequired(c => c.Department)    .WithMany(t => t.Courses)    .Map(m => m.MapKey("ChangedDepartmentID"));
      

Configuring a Foreign Key Name That Does Not Follow the Code First Convention

　　如果从属实体上的外键属性名不符合 Code First Conventions 的规范，意即从属实体上没有外键，你可以通过如下方式来指定

// Configuring a Foreign Key Name That Does Not Follow the Code First ConventionmodelBuilder.Entity
      
       ()     .HasRequired(c => c.Department)     .WithMany(d => d.Courses)     .HasForeignKey(c => c.SomeDepartmentID);
      

原文链接：

摘抄：http://www.cnblogs.com/panchunting/p/entity-framework-code-first-fluent-api-configuring-relationships.html