Code First Entity Framework Many-to-Many relationships

Question

Can someone point out where I've got wrong!!

I have created 2 simple classes, with many-to-many relationship. Works fine, all the tables are populated correctly. Except when I try and retrive any Students courses nothing is returned...

public partial class Student 
{

    public Student()
    {
        Courses = new HashSet<Course>();
    }

    public int StudentID { get; set; }

    [StringLength(50)] 
    [Required]
    public string FirstName { get; set;}

    [StringLength(50)]
    [Required]
    public string LastName {get; set;}

    public DateTime? Enroled {get;set;}

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


}

public class Course
{

    public Course()
    {
        Students = new HashSet<Student>();
    }
    public int CourseID { get; set; }

    [StringLength(30)]
    [Required]
    public string CourseTitle { get; set; }

    [StringLength(255)]
    public string CourseDesc { get; set; }

    public ICollection<Student> Students { get; set; }
}

    public ContextDB()
        : base (@"Data Source=.\SQLEXPRESS;Initial Catalog=CollegeDB;Integrated Security=True")
    {

    }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Course>().HasMany(c=>c.Students).WithMany(p => p.Courses)
            .Map(
            m=> {
                m.MapLeftKey("Course_CourseID");
                m.MapRightKey("Student_StudentID");
                m.ToTable("CourseStudents");
            });
    }

    public DbSet<Student> Students { get; set; }
    public DbSet<Course> Courses { get; set; }


}



          var students = db.Students.ToList();

            foreach (Student s in students)
            {
                Console.WriteLine("{0} {1}", s.FirstName, s.LastName);
                foreach (Course c in s.Courses.ToList())
                {
                    Console.WriteLine("{0}", c.CourseTitle);
                }
            }

Jayantha Lal Sirisena · Accepted Answer · 2012-03-21 12:07:27Z

8

If you are using lazy loading you need to defiine properties as virtual ,

 public virtual ICollection<Course> Courses { get; set; }

and if you want you can use eager loading,

var students = db.Students.Include(s=>s.Courses).ToList();

http://codetuner.blogspot.com/2011/07/entity-framework-differed-loading-lazy.html

answered Mar 21, 2012 at 12:07

Jayantha Lal Sirisena

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2 Comments

Slauma Over a year ago

@Karb: You can "accept" answers by clicking on the white check mark left to the answer to indicate that the answer did solve your problem.

lintmouse Over a year ago

You saved me an obscene amount of time with this. I forgot to make a couple collections virtual and didn't even think to check that.

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