Linq2SQL转换为外部联接

我有一个linq-to-sql查询，它具有多个联接。我想将其中一个转换为外部联接。问题是这个查询已经有一个from子句。如何将此查询转换为在OrderCertification表上使用左联接而不是内部联接？

以下查询不起作用（导致我的应用程序崩溃）：

var orderSummaries = from os in DbOrder.QueryOrderSummaries().Where(os => orders.Contains(os.OrderID))
   join o in dc.ORDERs on os.OrderID equals o.OrderID
   join oa in dc.ORDERADDRESSes on os.OrderID equals oa.OrderID
   join d in dc.vDoctors on o.DoctorID equals d.DoctorID
   join c in dc.ORDERCERTIFICATIONs on os.OrderID equals c.OrderID into oc
   from certification in oc.DefaultIfEmpty()
   select new BatchOrderItem {
      OrderSummary = os,
      Order = o,
      ShipTo = oa,
      Prescriber = d,
      CertificationContact = certification
    };