显式关联

场景

什么是显式关联呢?在面向对象理论中，关联用来描述两个类或者说两种对象)之间的关系，正如对象具有属性一样，关系也可能有属性。 这样的例子很多，比如老师与班级之间的“教学” (Teaching)关系，这是一个显式关联，因为一个老师在不同班级教授的课程可能不同。在面向对象理论中用关联类来(图中Teaching类)表示显式关联专统ORM框架不能直接支持显式关联的映射，必须把关联类(Teaching)改为实体类，并且定义两个关联分别指向Car和Wheel。这样虽然可以正确地向数据库写入数据，但模型的内涵却背离了现实世界，从根本上违反了面向对象理念。

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

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

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

     /// <summary>
     ///班级的授课
     /// </summary>
     private List<Teaching> _teachings;

     /// <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>
     ///班级的授课
     /// </summary>
     public List<Teaching> Teachings
     {
         get => _teachings;
         set => _teachings = value;
     }
 }

 /// <summary>
 ///教师
 /// </summary>
 public class Teacher
 {
     /// <summary>
     ///年龄
     /// </summary>
     private int _age;

     /// <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>
     ///教授班级
     /// </summary>
     private List<Teaching> _teachings;

     /// <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<Teaching> Teachings
     {
         get => _teachings;
         set => _teachings = value;
     }
 }

 /// <summary>
 ///表示授课关系
 /// </summary>
 public class Teaching
 {
     /// <summary>
     ///授课的班级Id
     /// </summary>
     private readonly int _classId;

     /// <summary>
     ///授课的教师ID
     /// </summary>
     private readonly int _teacherId;

     /// <summary>
     ///授课的班级
     /// </summary>
     private Class _class;

     /// <summary>
     ///教授的课程
     /// </summary>
     private string _course;

     /// <summary>
     ///授课的教师
     /// </summary>
     private Teacher _teacher;

     /// <summary>
     ///初始化授课关系
     /// </summary>
     /// <param name="course">所授课程</param>
     /// <param name="teacher">授课的教师</param>
     /// <param name="class">授课的班级</param>
     public Teaching(string course, Teacher teacher, Class @class)
     {
         _course = course;
         _teacher = teacher;
         _class = @class;
         _teacherId = teacher.TeacherId;
         _classId = @class.ClassId;
     }

     /// <summary>
     ///反持久化构造函数
     /// </summary>
     protected Teaching(int teacherId, int classId)
     {
         _teacherId = teacherId;
         _classId = classId;
     }

     /// <summary>
     ///教授的课程
     /// </summary>
     public string Course
     {
         get => _course;
         set => _course = value;
     }

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

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

     /// <summary>
     ///授课的教师ID
     /// </summary>
     public int TeacherId => _teacherId;

     /// <summary>
     ///授课的班级Id
     /// </summary>
     public int ClassId => _classId;
 }

模型配置

 /// <summary>
 ///显示关联配置
 /// </summary>
 public class ExplicitlyConfiguration : 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);

         //有独属于关系的属性 需要配置为显式关联型
         //配置显式关联型Teaching
         var teachingAssociation = modelBuilder.Association<Teaching>();
         //配置一个反持久化构造函数
         teachingAssociation.HasConstructor(typeof(Teaching).GetConstructor(
                 BindingFlags.Instance | BindingFlags.NonPublic,
                 null, new[] { typeof(int), typeof(int) }, null))
             //指示TeacherId ClassId已经在此构造函数内赋值 此处需要与反持久化构造函数参数顺序一致
             .Map(p => p.TeacherId).Map(p => p.ClassId).End();
         //配置关联端 班级关联端为Class这个属性 在关联表中Class的主键ClassId映射为ClassId
         teachingAssociation.AssociationEnd(p => p.Class).HasMapping("ClassId", "ClassId");
         //配置关联端 教师关联端为Teacher这个属性 在关联表中Teacher的主键TeacherId映射为TeacherId
         teachingAssociation.AssociationEnd(p => p.Teacher).HasMapping("TeacherId", "TeacherId");

         //配置班级实体型中的关联引用 左端就是关联型上自己类型的这一端 即Class 右端即为另外一端Teacher
         classEntity.AssociationReference(p => p.Teachings).HasLeftEnd("Class").HasRightEnd("Teacher");
         //配置教师实体型中的关联引用 左端就是关联型上自己类型的这一端 即Teacher 右端即为另外一端Class
         teacherEntity.AssociationReference(p => p.Teachings).HasLeftEnd("Teacher").HasRightEnd("Class");
     }
 }