Correct way to remove a many-to-many relationship via linq to sql?

Question

Let\'s say we have two tables with a many-to-many relationship:

public class Left{ /**/ }

public class Right{ /**/ }

public class LeftRight{ /**/ }
         


        

Answer 1

Take two, using expressions:

public static class EntitySetExtensions
{
  public static void UpdateReferences(
      this EntitySet refs,
      Expression> fkexpr,
      IEnumerable values)
    where FK : class
    where FKV : class
  {
    Func fkvalue = fkexpr.Compile();
    var fkmaker = MakeMaker(fkexpr);
    var fkdelete = MakeDeleter(fkexpr);

    var fks = refs.Select(fkvalue).ToList();
    var added = values.Except(fks);
    var removed = fks.Except(values);

    foreach (var add in added)
    {
      refs.Add(fkmaker(add));
    }

    foreach (var r in removed)
    {
      var res = refs.Single(x => fkvalue(x) == r);
      refs.Remove(res);
      fkdelete(res);
    }
  }

  static Func MakeMaker(Expression> fkexpr)
  {
    var me = fkexpr.Body as MemberExpression;

    var par = Expression.Parameter(typeof(FKV), "fkv");
    var maker = Expression.Lambda(
        Expression.MemberInit(Expression.New(typeof(FK)), 
          Expression.Bind(me.Member, par)), par);

    var cmaker = maker.Compile() as Func;
    return cmaker;
  }

  static Action MakeDeleter(Expression> fkexpr)
  {
    var me = fkexpr.Body as MemberExpression;
    var pi = me.Member as PropertyInfo;

    var par = Expression.Parameter(typeof(FK), "fk");
    var maker = Expression.Lambda(
        Expression.Call(par, pi.GetSetMethod(), 
          Expression.Convert(Expression.Constant(null), typeof(FKV))), par);

    var cmaker = maker.Compile() as Action;
    return cmaker;
  }
}

Now the usage is uber simple! :)

Left entity = ...;
IEnumerable rights = ...;

entity.LeftRights.UpdateReferences(x => x.Right, rights);

The first expression is now used to establish the 'relationship'. From there I can infer the 2 previously required delegates. Now no more :)

Important:

To get this to work properly in Linq2Sql, you need to mark the associations from intermediary table with 'DeleteOnNull="true"' in the dbml file. This will break the designer, but still works correctly with SqlMetal.

To unbreak the designer, you need to remove those additional attributes.