Figure 5. Set At Least One Column's IsID Property to True The current DLinq implementation doesn't detect fields that comprise a source table's composite primary key as "identity key" values for DLinqClasses. If you don't have at least one DLinqProperty (field) with an IsID property value of True, your project throws a "The type 'Order_Detail' has no identity key" exception. For this example, start by setting the IsID property value of the Order_Detail entity's OrderID DLinqProperty to True.