Entity Framework Code First （五）Fluent API - 配置关系

川长思鸟来 2021-09-30 23:42 260阅读 0赞

# [Entity Framework Code First （五）Fluent API - 配置关系][Entity Framework Code First _Fluent API -] #

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

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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; }
        }

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***EntityTypeConfiguration<TEntityType>***

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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>* ###

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### *OptionalNavigationPropertyConfiguration<TEntityType, TTargetEntityType>* ###

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### *ManyNavigationPropertyConfiguration<TEntityType, TTargetEntityType>* ###

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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）

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// 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");
        });

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### *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

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// 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 });

![复制代码][copycode.gif]

### *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);

转载于:https://www.cnblogs.com/sjqq/p/7709619.html

[Entity Framework Code First _Fluent API -]: http://www.cnblogs.com/panchunting/p/entity-framework-code-first-fluent-api-configuring-relationships.html
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