title: DeleteOnNull (Association attribute) for Linq to SQL in custom class?
body: <p>Is it possible to add the "DeleteOnNull=true" on a custom class instead of modifying the DBML (generated) class directly?</p>
  
  <p>For example, let's say this is a part of my generated dbml class:</p>
  
  <pre><code>[Table(Name="OrderDetails")]
  public partial class OrderDetail : 
         INotifyPropertyChanging, INotifyPropertyChanged
  {
      // deleted for brevity
  
      [Association(Name="Order_OrderDetail", 
           Storage="_Order", ThisKey="OrderId", 
           OtherKey="OrderId", IsForeignKey=true, DeleteOnNull=true)]
      public Order Order 
      {
          get { /* deleted */ }
          set { /* deleted */ }
      }
  }
  </code></pre>
  
  <p>So is it possible to put the "DeleteOnNull=true" on a separate class? Is it is? How? I have tried the following without any luck:</p>
  
  <pre><code>[MetadataType(typeof(OrderDetailMetadata))]
  public partial class OrderDetail {
      internal sealed class OrderDetailMetadata
      {
          [Association(DeleteOnNull = true)]
          public object Order;
      }
  }
  </code></pre>