Simple Delete in Entity Framework using WCF (many to many relationship)

I have a simple database model containing 3 Tables : Companies, Categories and CompanyCategories (which is a relation table with only 2 FK : CompanyID and CategoryID).

My edmx model it only shows Companies and Categories tables (CompanyCategories is somehow hidded since its a simple many to many relationship table).

In the WCF service, I have a GetDatabase() function that returns all the database objets wrapped in one big custom object :

[OperationContract]
public FullDatabase GetDatabase()
{
   DBEntities context = new DBEntities ();

   FullDatabase mydb = new FullDatabase();
   mydb.Companies = context.Companies.ToList();
   mydb.Categories = context.Categories.ToList();

   return mydb;
}

[OperationContract]
public FullDatabase UpdateDatabase(FullDatabase db)
{
    // Here is my problem when removing a category from a company on 
    // the client its been brought back in my db object 
}

class FullDatabase()
{
   List<Comp开发者_如何学运维any> Companies;
   List<Category> Categories;
}

On the client now, I use GetDatabaseAsync() to retrieve the database in a _FullDB variable. Now using that variable I tried the following :

  // Adding a category like that Works well
  Company c = _FullDB.Companies.First();
  c.Categories.Add(_FullDB.Categories.First());
  wcfServiceClientObject.UpdateDatabaseASync(_FullDB);

  .....

  // Removing a category, doesn't work though :
  Company c = _FullDB.Companies.First();
  c.Categories.Remove(_FullDB.Categories.First());
  wcfServiceClientObject.UpdateDatabaseASync(_FullDB);

    // here my c.Categories.Count is updated correctly to delete the item
    // but when on the server after (in the UpdateDatabase function) the item 
    // I deleted is still there

I really dont understand why the Add would work but not the Remove.

Finally found the problem. Now it works, but I'm not sure it is the best way to do it.

When removing the category from the company, I also had to also remove the company from the category...

Company comp = _FullDB.Companies.First();
Category cat = _FullDB.Categories.First();

comp.Categories.Remove(cat);
cat.Companies.Remove(comp);

wcfServiceClientObject.UpdateDatabaseASync(_FullDB);