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类之间有多个关联

场景​

很多ORM框架都支持关联的持久化与反持久化，比如EF提供了为关联建模的API，我们看一个例子.老师和班级之间存在"授课"的关联，一个老师可以为多个开放系统授讯，一个班也会有多个授课老师。用EF为这个关联建模，代码如下: 建横之后EF会自动跟踪这类关联实例的变化从而实施持久化;同时也可以反持久化该关联，例如加载Teacher类的属性TaughtClasses、加载Class类的属性Teachers 大多数情况下，两个类之间关联的数量不会超过1个，但也会有例外。我们仍然以都和班级的关系为例，两者之间除了有"授课"关联，在学校组织考试的时候还会存在"监考"的关联。那么，能够使用持久化框架同时处理“授课”和"监考"这两个关联吗?

http://indexmp4.suiyiyun.cn/img/image-20231031200958232.png

解决方案​

实体模型​

 /// <summary>
 ///班级
 /// </summary>
 public class Class
 {
     /// <summary>
     ///班级ID
     /// </summary>
     private int _classId;

     /// <summary>
     ///学院
     /// </summary>
     private string _college;

     /// <summary>
     ///班级的监考教师
     /// </summary>
     private List<Teacher> _invigilateTeachers;

     /// <summary>
     ///编号
     /// </summary>
     private byte _number;

     /// <summary>
     ///专业
     /// </summary>
     private string _profession;

     /// <summary>
     ///学生数
     /// </summary>
     private int _studentCount;

     /// <summary>
     ///班级的任课教师
     /// </summary>
     private List<Teacher> _teachers;

     /// <summary>
     ///班级ID
     /// </summary>
     public int ClassId
     {
         get => _classId;
         set => _classId = value;
     }

     /// <summary>
     ///专业
     /// </summary>
     public string Profession
     {
         get => _profession;
         set => _profession = value;
     }

     /// <summary>
     ///编号
     /// </summary>
     public byte Number
     {
         get => _number;
         set => _number = value;
     }

     /// <summary>
     ///学院
     /// </summary>
     public string College
     {
         get => _college;
         set => _college = value;
     }

     /// <summary>
     ///学生数
     /// </summary>
     public int StudentCount
     {
         get => _studentCount;
         set => _studentCount = value;
     }

     /// <summary>
     ///班级的任课教师
     /// </summary>
     public List<Teacher> Teachers
     {
         get => _teachers;
         set => _teachers = value;
     }

     /// <summary>
     ///班级的监考教师
     /// </summary>
     public List<Teacher> InvigilateTeachers
     {
         get => _invigilateTeachers;
         set => _invigilateTeachers = value;
     }
 }
 /// <summary>
 ///表示监考关系
 /// </summary>
 public class Invigilating
 {
     /// <summary>
     ///班级
     /// </summary>
     private Class _class;

     /// <summary>
     ///教师
     /// </summary>
     private Teacher _teacher;

     /// <summary>
     ///教师
     /// </summary>
     public Teacher Teacher
     {
         get => _teacher;
         set => _teacher = value;
     }

     /// <summary>
     ///班级
     /// </summary>
     public Class Class
     {
         get => _class;
         set => _class = value;
     }
 }
 /// <summary>
 ///教师
 /// </summary>
 public class Teacher
 {
     /// <summary>
     ///年龄
     /// </summary>
     private int _age;

     /// <summary>
     ///教授班级
     /// </summary>
     private List<Class> _classes;

     /// <summary>
     ///监考的班级
     /// </summary>
     private List<Class> _invigilateClasses;

     /// <summary>
     ///姓名
     /// </summary>
     private string _name;

     /// <summary>
     ///职业
     /// </summary>
     private string _profession;

     /// <summary>
     ///性别
     /// </summary>
     private bool _sex;

     /// <summary>
     ///教师ID
     /// </summary>
     private int _teacherId;

     /// <summary>
     ///教师ID
     /// </summary>
     public int TeacherId
     {
         get => _teacherId;
         set => _teacherId = value;
     }

     /// <summary>
     ///姓名
     /// </summary>
     public string Name
     {
         get => _name;
         set => _name = value;
     }

     /// <summary>
     ///职业
     /// </summary>
     public string Profession
     {
         get => _profession;
         set => _profession = value;
     }

     /// <summary>
     ///性别
     /// </summary>
     public bool Sex
     {
         get => _sex;
         set => _sex = value;
     }

     /// <summary>
     ///年龄
     /// </summary>
     public int Age
     {
         get => _age;
         set => _age = value;
     }

     /// <summary>
     ///教授班级
     /// </summary>
     public List<Class> Classes
     {
         get => _classes;
         set => _classes = value;
     }

     /// <summary>
     ///监考的班级
     /// </summary>
     public List<Class> InvigilateClasses
     {
         get => _invigilateClasses;
         set => _invigilateClasses = value;
     }
 }
 /// <summary>
 ///表示授课关系
 /// </summary>
 public class Teaching
 {
     /// <summary>
     ///班级
     /// </summary>
     private Class _class;

     /// <summary>
     ///教师
     /// </summary>
     private Teacher _teacher;

     /// <summary>
     ///教师
     /// </summary>
     public Teacher Teacher
     {
         get => _teacher;
         set => _teacher = value;
     }

     /// <summary>
     ///班级
     /// </summary>
     public Class Class
     {
         get => _class;
         set => _class = value;
     }
 }

模型配置​

public class CustomImplicitConfiguration : MySqlContextConfigProvider
{
    /// <summary>由派生类实现，获取数据库连接字符串。</summary>
    protected override string ConnectionString => Configuration.MySqlConnectionString;

    /// <summary>获取一个值，该值指示是否启用存储结构映射</summary>
    protected override bool EnableStructMapping => true;

    /// <summary>使用指定的建模器创建对象数据模型。</summary>
    /// <param name="modelBuilder"></param>
    protected override void CreateModel(ModelBuilder modelBuilder)
    {
        //配置班级实体型
        var classEntity = modelBuilder.Entity<Class>();
        //配置主键
        classEntity.HasKeyAttribute(p => p.ClassId).HasKeyIsSelfIncreased(true);

        //配置教师实体型
        var teacherEntity = modelBuilder.Entity<Teacher>();
        //配置主键
        teacherEntity.HasKeyAttribute(p => p.TeacherId).HasKeyIsSelfIncreased(true);

        //两个类之间有多种隐式关系 那就配置两个不同的隐式关联型
        //配置教师与班级间的表示教授关系的关联型
        //注意此处的Teaching类为定义的隐式关联型类 类内除了关联端的引用外不应该存在属性
        var classTeacherAssociation1 = modelBuilder.Association<Teaching>();
        //此处关联型上无属性为隐式关联型 故需要指定HasVisible(false)
        classTeacherAssociation1.HasVisible(false);
        //配置关联端 Class端为Class 在关联表中Class的主键ClassId映射为ClassId
        classTeacherAssociation1.AssociationEnd(p => p.Class).HasMapping("ClassId", "ClassId");
        //配置关联端 Teacher端为Teacher 在关联表中Teacher的主键TeacherId映射为TeacherId
        classTeacherAssociation1.AssociationEnd(p => p.Teacher).HasMapping("TeacherId", "TeacherId");
        //多对多 独立映射表
        classTeacherAssociation1.ToTable("Teaching");

        //配置教师与班级间的表示监考关系的关联型
        //注意此处的Invigilating类为定义的隐式关联型类 类内除了关联端的引用外不应该存在属性
        var classTeacherAssociation2 = modelBuilder.Association<Invigilating>();
        //此处关联型上无属性为隐式关联型 故需要指定HasVisible(false)
        classTeacherAssociation2.HasVisible(false);
        //配置关联端 Class端为Class 在关联表中Class的主键ClassId映射为ClassId
        classTeacherAssociation2.AssociationEnd(p => p.Class).HasMapping("ClassId", "ClassId");
        //配置关联端 Teacher端为Teacher 在关联表中Teacher的主键TeacherId映射为TeacherId
        classTeacherAssociation2.AssociationEnd(p => p.Teacher).HasMapping("TeacherId", "TeacherId");
        //多对多 独立映射表
        classTeacherAssociation2.ToTable("Invigilating");

        //配置关联引用 此处有两种关系 所以每个类中也相应的有两个关联引用
        //注意此处的关联引用使用了指定关联型的关联引用配置方法 AssociationReference<T>
        //左端为自己 即Class 右端为 Teacher
        classEntity.AssociationReference<Teaching>("Teachers", true).HasLeftEnd("Class").HasRightEnd("Teacher");
        classEntity.AssociationReference<Invigilating>("InvigilateTeachers", true).HasLeftEnd("Class")
            .HasRightEnd("Teacher");
        //左端为自己 即Teacher 右端为 Class 
        teacherEntity.AssociationReference<Teaching>("Classes", true).HasLeftEnd("Teacher").HasRightEnd("Class");
        teacherEntity.AssociationReference<Invigilating>("InvigilateClasses", true).HasLeftEnd("Teacher")
            .HasRightEnd("Class");
    }
}