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<Enrollment> Enrollments { get; set; }
} public class Department
{
public Department()
{
this.Courses = new HashSet<Course>();
}
// 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<Course> 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<Enrollment> Enrollments { get; set; }
public virtual ICollection<Instructor> 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<Course> 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<TKey>(Expression<Func<TDependentEntityType, TKey>> foreignKeyExpression);
其它两个类主要包含方法 Map ，如下
ForeignKeyNavigationPropertyConfiguration
public CascadableNavigationPropertyConfiguration Map(Action<ForeignKeyAssociationMappingConfiguration> configurationAction);
ManyToManyNavigationPropertyConfiguration<TEntityType, TTargetEntityType>
public ManyToManyNavigationPropertyConfiguration<TEntityType, TTargetEntityType> Map(Action<ManyToManyAssociationMappingConfiguration> configurationAction);
public ManyToManyNavigationPropertyConfiguration<TEntityType, TTargetEntityType> MapToStoredProcedures();
public ManyToManyNavigationPropertyConfiguration<TEntityType, TTargetEntityType> MapToStoredProcedures(Action<ManyToManyModificationStoredProceduresConfiguration<TEntityType, TTargetEntityType>> 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 relationship
modelBuilder.Entity<OtherInfo>()
.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 relationship
modelBuilder.Entity<Address>()
.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 relationship
modelBuilder.Entity<Student>()
.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 relationship
modelBuilder.Entity<Course>()
.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 relationship
modelBuilder.Entity<Course>()
.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 relationship
modelBuilder.Entity<Address>()
.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<OneToManyCascadeDeleteConvention>()
modelBuilder.Conventions.Remove<ManyToManyCascadeDeleteConvention>()
Code First Fluent API 的方式如下
// Cascade Delete
modelBuilder.Entity<Course>()
.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 key
modelBuilder.Entity<Department>()
.HasKey(d => new { d.DepartmentID, d.Name }); // Composite foreign key
modelBuilder.Entity<Course>()
.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 Model
modelBuilder.Entity<Course>()
.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 Convention
modelBuilder.Entity<Course>()
.HasRequired(c => c.Department)
.WithMany(d => d.Courses)
.HasForeignKey(c => c.SomeDepartmentID);