Question

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

public class Left{ /**/ }

public class Right{ /**/ }

public class LeftRight{ /**/ }

is the following sufficient to unhook these records (ignore the possibility of more than one relationship or no relationship defined)?

public void Unhook(Left left, Right right){
  var relation = from x in Left.LeftRights where x.Right == right;
  left.LeftRrights.Remove(relation.First());
  Db.SubmitChanges();
}

Or do I have to do it on both parts? What’s required here?

Answer 1

Here is a ‘little’ extension method I wrote to simplify this problem:

  public static class EntitySetExtensions
  {
    public static void UpdateReferences<FK, FKV>(
        this EntitySet<FK> refs,
        Func<FK, FKV> fkvalue,
        Func<FKV, FK> fkmaker,
        Action<FK> fkdelete,
        IEnumerable<FKV> values)
      where FK : class
      where FKV : class
    {
      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);
      }
    }
  }

It could probably be improved, but it has served me well 🙂

Example:

Left entity = ...;
IEnumerable<Right> rights = ...;

entity.LeftRights.UpdateReferences(
 x => x.Right, // gets the value
 x => new LeftRight { Right = x }, // make reference
 x => { x.Right = null; }, // clear references
 rights);

Algorithm description:

Suppose A and B is many-to-many relationship, where AB would be the intermediary table.

This will give you:

class A { EntitySet<B> Bs {get;} }
class B { EntitySet<A> As {get;} }
class AB { B B {get;} A A {get;} }

You now have an object of A, that reference many B’s via AB.

1. Get all the B from A.Bs via ‘fkvalue’.
2. Get what was added.
3. Get what was removed.
4. Add all the new ones, and construct AB via ‘fkmaker’.
5. Delete all the removed ones.
6. Optionally, remove other referenced objects via ‘fkdelete’.

I would like to improve this by using Expression instead, so I could ‘template’ the method better, but it would work the same.