实体框架代码中的许多关系首先使用“虚拟”关键字相互访问

2 回答

• 11

  public class Student
  {
      public virtual int StudentId { get; set; }
      public virtual string StudentName { get; set; }
  
      public virtual ICollection<Enrollment> Enrollments { get; set; }
  }
  
  public class Course
  {
      public virtual int CourseId { get; set; }
      public virtual string CourseName { get; set; }
  
      public virtual ICollection<Enrollment> Enrollments { get; set; }
  }
  
  public class Enrollment
  {
      public virtual int StudentId { get; set; }
      public virtual int CourseId { get; set; }
      public virtual string Grade { get; set; }
  
      public virtual Student Student { get; set; }
      public virtual Course Course { get; set; }
  }
  
  public class ManyMany : DbContext
  {
      public DbSet<Student> Students { get; set; }
      public DbSet<Course> Courses { get; set; }
      public DbSet<Enrollment> Enrollments { get; set; }
  
      protected override void OnModelCreating(DbModelBuilder modelBuilder)
      {
          modelBuilder.Entity<Student>()
              .HasKey(student => student.StudentId);
          modelBuilder.Entity<Course>()
              .HasKey(course => course.CourseId);
          modelBuilder.Entity<Enrollment>()
              .HasKey(enrollment => new { enrollment.StudentId, enrollment.CourseId } );
  
          modelBuilder.Entity<Student>()
              .HasMany(student => student.Enrollments)
              .WithRequired(enrollment => enrollment.Student)
              .HasForeignKey(enrollment => enrollment.StudentId);
          modelBuilder.Entity<Course>()
              .HasMany(course => course.Enrollments)
              .WithRequired(enrollment => enrollment.Course)
              .HasForeignKey(enrollment => enrollment.CourseId);
      }
  }
  

  关于这个问题的老问题，答案和更多信息：Entity Framework CodeFirst many to many relationship with additional information .

  编辑：用法示例：

  var context = new ManyMany();
  
      var physicsCourse = new Course() { CourseName = "Physics" };
      var mathCourse = new Course() { CourseName = "Math" };
  
      var studentJohn = new Student() { StudentName = "John Doe" };
      var studentJane = new Student() { StudentName = "Jane Doe" };
  
      var physicsCourseEnrollmentJohn = new Enrollment() { Student = studentJohn, Course = physicsCourse };
      var mathCourseEnrollmentJohn = new Enrollment() { Student = studentJohn, Course = mathCourse };
      var physicsCourseEnrollmentJane = new Enrollment() { Student = studentJane, Course = physicsCourse };
  
      context.Courses.Add(physicsCourse);
      context.Courses.Add(mathCourse);
      context.Students.Add(studentJohn);
      context.Students.Add(studentJane);
  
      studentJohn.Enrollments.Add(physicsCourseEnrollmentJohn);
      studentJohn.Enrollments.Add(mathCourseEnrollmentJohn);
      studentJane.Enrollments.Add(physicsCourseEnrollmentJane);
  
      physicsCourse.Enrollments.Add(physicsCourseEnrollmentJohn);
      mathCourse.Enrollments.Add(mathCourseEnrollmentJohn);
      physicsCourse.Enrollments.Add(physicsCourseEnrollmentJane);
  
      context.Enrollments.Add(physicsCourseEnrollmentJohn);
      context.Enrollments.Add(mathCourseEnrollmentJohn);
      context.Enrollments.Add(physicsCourseEnrollmentJane);
  
      context.SaveChanges();
  
      var johnsEnrollments = context.Students.Where(student => student.StudentId == studentJohn.StudentId).Single().Enrollments;
      MessageBox.Show(string.Format("Student John has enrolled in {0} courses.", johnsEnrollments.Count));
      var janesEnrollments = context.Students.Where(student => student.StudentId == studentJane.StudentId).Single().Enrollments;
      MessageBox.Show(string.Format("Student Jane has enrolled in {0} courses.", janesEnrollments.Count));
  

  回复于 2024-05-14T01:37:48+08:00
• 5

  实体框架可以't automatically determine '多对多'关系，因为它们是在SQL的其他表的帮助下表达的（在您的情况下是 Enrollment 表） . 您可以直接在 OnModelCreating 方法中指定映射：

  public class YourDbContext : DbContext
  {
      ....
  
      protected override void OnModelCreating(DbModelBuilder modelBuilder)
      {
          modelBuilder.Entity<Student>().HasMany(x => x.Courses).WithMany(x => x.Students)
              .Map(m =>
              {
                  m.ToTable("Enrollment"); // Relationship table name
                  m.MapLeftKey("StudentID"); // Name of column for student IDs
                  m.MapRightKey("CourseID"); // Name of column for course IDs
              });
      }
  }
  

  另外，请注意，如果实体有许多其他实体，请使用集合进行关系：

  public class Student
  {
      ....
      public virtual ICollection<Course> Courses { get; set; } // Many courses
  }
  
  public class Course
  {
      ....
      public virtual ICollection<Student> Students { get; set; } // Many students
  }
  

  回复于 2024-05-14T01:37:48+08:00