c# - Entity Framework Code First 中的多对多关系和使用 "virtual"关键字相互访问

Question

这段摘录代码成功地创建了一个多对多关系，其中包含一个包含额外数据的显式 Junction 表。

问题:我希望能够从学生访问类(class)，反之亦然，
(因此注释的虚拟属性。但如果我取消注释它，它会导致错误(见下文))

如果我没有显式创建联结表(没有其他数据)，virtual 关键字会起作用，因为 EF 会按照约定创建联结表。

问题:

如何让学生访问类(class)而不通过注册？或者那是不可能的？如果不可能，那么最好的方法是什么？

(EF 和 C# 初学者)

    public class Student
    {
        [Key]
        public int StudentId { get; set; }
        public string StudentName { get; set; }

        //public virtual Course Courses { get; set; }
    }

    public class Course
    {
        [Key]
        public int CourseId { get; set; }
        public string CourseName { get; set; }

        //public virtual Student Students { get; set; }
    }

    public class Enrollment
    {
        [Key]
        public int EnrollmentId { get; set; }
        public Student Student { get; set; }
        public Course Course { get; set; }
        public string Grade { get; set; }
    }

    public class ManyMany : DbContext, IContext
    {
        public DbSet<Student> Students { get; set; }
        public DbSet<Course> Courses { get; set; }
        public DbSet<Enrollment> Enrollments { get; set; }

        public void Run()
        {
            Database.SetInitializer(new DropCreateDatabaseAlways<ManyMany1>());
            this.Courses.Add(new Course() {CourseName = "English"});
            this.SaveChanges();
        }
    }

当我取消注释公共(public)虚拟......

错误:“无法确定‘EF.Course’和‘EF.Student’类型之间关联的主体端。必须使用关系流畅的 API 或数据注释。”

Answer 1

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);
    }
}

关于此的较旧问题，答案和更多信息在这里:Entity Framework CodeFirst many to many relationship with additional information .

编辑:使用示例:

    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));