Save a relation with between two entities an N-N association

Question

I've a Entity Framework 4.0, with poco object. the edmx model file is generated from the database.

This datacontext is accessed through WCF service, it's only mean that I receive some objects and I need to attach them to the current datacontext(or reload them with the key correspondance).

Everything seems to work fine, except for one case:

I've a N-N relationship between two table, so I've an association table, without any field other than ID of two tables:

LINQ transform this into the following schema, this seems to be right.

When I retrieve data, there is no problem, data I've inserted myself in the Right_group are correctly transformed into "new object in my collection of Rights/Groups".

But if I try to modify something and save, it doesn't work

 public void SaveRights(Group group, List<Rights> rights){
     //here, group and rights are objects attached to the database
     group.Rights.Clear();
     group.Rights.AddRange(rights);
     _dataContext.SaveChanges();
 }

So my question is: How to save this "relationship" of two objects ? Thank you!

Answer 1

If you want to avoid loading the objects from the database first you can do it like this(Code taken from one of my aplications so you will have to adapt it):

    public void AddAndRemovePersons(int id, int[] toAdd, int[] toDelete)
    {
        var mailList = new MailList { ID = id, ContactInformations = new List<ContactInformation>() };        
        this.db.MailLists.Attach(mailList);

        foreach (var item in toAdd)
        {
            var ci = new ContactInformation { ID = item };
            this.db.ContactInformations.Attach(ci);
            this.db.ObjectStateManager.ChangeRelationshipState(mailList, ci, ml => ml.ContactInformations, System.Data.EntityState.Added);
        }

        foreach (var item in toDelete)
        {

            var ci = new ContactInformation { ID = item };
            this.db.ContactInformations.Attach(ci);
            this.db.ObjectStateManager.ChangeRelationshipState(mailList, ci, ml => ml.ContactInformations, System.Data.EntityState.Deleted);
        }
    }

I found deleting the relationship as hard as creating it so I left that code in there. One thing about this solution is that both the maillist and the contacts exist prior to this function being run. I attach them to make the state manager track them.

If you are adding new objects that you also want to save you would use the

this.db.MailLists.AddObject(you new item here)

I hope that helps!