Entity Framework Code First （五）Fluent API - 配置关系 转载 https://www.cnblogs.com/panchunting/p/entity-framework-code-first-fluent-api-configuring-relationships.html

　　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);

原文链接：http://msdn.microsoft.com/en-us/data/jj591620