How to create many to many relationships in Entity Framework Core

Question

I had some research on the internet and found this blog post on learn Entity Framework, but after I code it, it seemed it wasn't what I wanted.

As well as the example on the blog, I have Students and Courses . But, I don't want my courses to know whether they are related to courses or not (I wouldn't bother either). But, I want each student to have n courses and that each course could be assigned to more than one student.

public class Student
{
    public int id {get; set;}
    public string name {get; set;}

    public ICollection<Course> courses {get;set;} //or another proper container abstraction
}

public class Course
{
   public int id {get; set}
   public string name {get; set;}
}

So I would query Student to know its courses. I implemented this, created a migration, run it on a SQL Server database and implemented a little ASP.NET Core API, with CRUD methods. When I added a second Student that used the same Course , the first Student loses its Course .

Then I implemented the solution the blog suggests:

public class StudentCourse
{
    public int student_id {get;set; }
    public Student student {get; set; } 
    public int course_id {get; set; }
    public Course course {get; set; }
}

and add to DbContext this piece of code:

modelBuilder.Entity<StudentCourse>().HasKey(sc => new { sc.SId, sc.CId });

modelBuilder.Entity<StudentCourse>()
    .HasOne<Student>(sc => sc.Student)
    .WithMany(s => s.StudentCourses)
    .HasForeignKey(sc => sc.SId);


modelBuilder.Entity<StudentCourse>()
    .HasOne<Course>(sc => sc.Course)
    .WithMany(s => s.StudentCourses)
    .HasForeignKey(sc => sc.CId);

And Student and Course would each have ICollection<StudentCourse> as members. The problem I found with this approach is that I can't create easily an instance of Student on my code, because the Student itself is a member of its member StudentCourse .

Answer 1

Look in your database. This

public ICollection<Course> courses {get;set;}

Will create a direct FK relationship on Course referencing Student. You're not using your StudentCourse entity at all. Should be

public class Student
{
    public int id {get; set;}
    public string name {get; set;}

    public virtual ICollection<StudentCourse> Courses {get;} = new HashSet<StudentCource>();
}