实体框架代码优先中的许多许多关系；虚拟的“；关键字来访问对方

本文关键字：虚拟 关键字 访问 代码 框架 实体 许多许 关系 | 更新日期: 2023-09-27 18:29:41

此摘录代码成功创建了一个多关系，其中包含一个包含额外数据的显式Junction表。

问题：我希望能够访问Student的课程，反之亦然，
（因此是注释的虚拟属性。但如果我取消注释，它会导致错误（见下文））

如果我没有显式地创建一个连接表（没有额外的数据），那么virtual关键字仍然有效，因为EF按照约定创建了一个连接表格。

问题：

如何让学生在不进行注册的情况下访问课程？或者这是不可能的？如果这不可能，那么最好的办法是什么

（EF和C#的初学者）

    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();
        }
    }

当我不服从公共虚拟。。。

错误："无法确定类型'EF.Curse'和'EF.Student'之间的关联的主体端。必须使用关系fluent API或数据批注显式配置此关联的主体一端。"

实体框架代码优先中的许多许多关系；虚拟的“；关键字来访问对方

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);
    }
}

关于这一点的老问题，在这里回答和更多信息：实体框架代码第一个多对多关系和其他信息。

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));

Entity Framework无法自动确定"多对多"关系，因为它们是在SQL中的附加表（在您的情况下是Enrollment表）的帮助下表达的。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
            });
    }
}

此外，请注意，如果一个实体有许多其他实体，则使用集合表示关系：

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