Many-many relationship in Entity Framework Code First and using the “virtual” keyword to access each other

Question

This excerpt code successfully creates a many-many relationship with an explicit Junction table that has additional data within it.

PROBLEM:

Answer 1

Entity Framework can't automatically determine 'many-to-many' relations because they are expressed with the help of additional tables in SQL (in your case it is Enrollment table). You can specify mappings directly in OnModelCreating method:

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
            });
    }
}

Also, take a note that if an entity have many other entities, use collection for relationship:

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

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

Answer 2

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);
    }
}

Older question concerning this, answer and more info here: Entity Framework CodeFirst many to many relationship with additional information.

EDIT: Usage example:

    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));