Insert records in many to many not working with Entity Framework

Question

I want to insert 2 new records in their tables and also to create the relation many to many.

There are a lot of questions with this topic, I tried many of the answers and now I have no idea why my code is not working.

Please help me!

This is the code:

class Program
{
    static void Main(string[] args)
    {
        MyContext db = new MyContext();

        var st1 = new Student() { Name = "Joe" };
        var c1 = new Course() { Name = "Math" };

        db.Courses.Attach(c1);
        st1.Courses.Add(c1);
        db.Students.Add(st1);         

        db.SaveChanges();            
    }       
}
public class Student
{
 public Student()
    {
        Courses = new HashSet<Course>();
    }
    public int Id { get; set; }
    public string Name { get; set; }
    public virtual ICollection<Course> Courses { get; set; }
}
public class Course
{
    public int Id { get; set; }
    public string Name { get; set; }
    public virtual ICollection<Student> Students { get; set; }
}
public class MyContext : DbContext
{
    public DbSet<Student> Students { get; set; }
    public DbSet<Course> Courses { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Student>()
            .HasMany(p => p.Courses)
            .WithMany(d => d.Students)
            .Map(t =>
            {
                t.MapLeftKey("studentId");
                t.MapRightKey("courseid");
                t.ToTable("StudentCourse");
            });

        base.OnModelCreating(modelBuilder);
    }

}

Edit: Like sugested, I initialized the Courses:

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

and now I get this error on db.SaveChanges();

An error occurred while saving entities that do not expose foreign key properties for their relationships. The EntityEntries property will return null because a single entity cannot be identified as the source of the exception. Handling of exceptions while saving can be made easier by exposing foreign key properties in your entity types. See the InnerException for details.

Possible duplicate of [What is a NullReferenceException and how do I fix it?](http://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-and-how-do-i-fix-it) — Eugene Podskal, Feb 27 '16 at 11:43
Your `st1` is not an EF wrapper - it is just the object of type `Student`, and `Student` class doesn't initialize its `Courses` collection. — Eugene Podskal, Feb 27 '16 at 11:44
Initilialize the `Courses` collection with some `new List()` or whatever. — Eugene Podskal, Feb 27 '16 at 11:46

Gert Arnold · Accepted Answer · 2016-02-27T22:23:43.357

1

You're apparently trying to add a new Student to the database and associate it to an existing Course. The problem is that you attach a new Course entity to the context without a proper primary key.

It certainly is a good idea to use a so-called stub entity here, because it saves a roundtrip to the database to fetch an existing Course, but the primary key is required for EF to create a correct association record. It's even the only property you need to set in this Course stub:

var st1 = new Student() { Name = "Joe" };
var c1 = new Course() { CourseId = 123 };

db.Courses.Attach(c1);
st1.Courses.Add(c1);
db.Students.Add(st1);

If you want to add a new course and a new student, you should Add both of them:

db.Courses.Add(c1);
st1.Courses.Add(c1);
db.Students.Add(st1);

edited Feb 27 '16 at 22:23

answered Feb 27 '16 at 22:12

Gert Arnold

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Both values (st1, c1) are new values, does not exist in database, and Yes, I was trying to insert in on trip this 2 new values in all 3 tables. Is it posible? – Lucian Bumb Feb 27 '16 at 22:18
Now is working :), i had tried a lot of combination, only this on no :) – Lucian Bumb Feb 27 '16 at 22:24
@Arnold can you please edit your answer with the comment, because I mark your answer as accepted. Thank you – Lucian Bumb Feb 27 '16 at 22:26
I already did. I just show both possibilities to make the answer more applicable for future visitors. – Gert Arnold Feb 27 '16 at 22:27

Dan · Answer 2 · 2016-02-27T12:22:45.010

Your code isn't initialising the ICollection object in either class.

public class Student
{
    private Student()
    {
        Courses = new List<Course>();
    }

    public int Id { get; set; }
    public string Name { get; set; }
    public virtual ICollection<Course> Courses { get; set; }
}

Edit

Try changing your modelBuilder code to the following

modelBuilder.Entity<Student>()
            .HasMany<Course>(s => s.Courses)
            .WithMany(c => c.Students)
            .Map(cs =>
                    {
                        cs.MapLeftKey("StudentRefId");
                        cs.MapRightKey("CourseRefId");
                        cs.ToTable("StudentCourse");
                    });

Code sample taken directly from:

http://www.entityframeworktutorial.net/code-first/configure-many-to-many-relationship-in-code-first.aspx

I got the same error 'new List();' 0 which I posted in the edit — Lucian Bumb, Feb 27 '16 at 12:16
I dropped the database, and recreated with your suggested changes in configuration, and gor the same error, like the one posted in my edit. — Lucian Bumb, Feb 27 '16 at 13:15

Insert records in many to many not working with Entity Framework

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