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().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 Courses { get; set; } // Many courses
}

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