We went ahead and put the question together with diagram and SQL statements in the attached document. I am sure this is possible to build in LLBLGen but we cant seem to figure out how.

To which field does the PurchaseOrderId in AcountPayableDetail point to?

The doc says it's not using a PK on the PK side but a UC, which means the relationship isn't recognized. As the relationship isn't recognized you can't map a reference/collection (field) on the relationship, so you can't use a prefetch path for this relationship (edge).

I wonder why the fk in the DB points to a UC. The whole AccountsPayableDetail table is odd to me as well.

We would like to bring AccountsPayable entity with its related AccountPayableDetails colletion

This doesn't make sense to me as well: the diagram doesn't show any direct relationship between the two, even through supertypes. AccountPayableDetail is related to AccountTransactionDetail, but that isn't directly related to AccountsPayable.

Thanks for confirming that our workaround idea wouldn't work. Your answers confirm my suspicion. In short, we can't get there from here. The dev working on this was literally interested in "grafting" a SuperType/SubType graph onto another one which, as you saw, is not directly related. The primary purpose for asking this was the notion that he could work on the entire graph in one shot. We certainly don't have to do that way, he can load two graphs and work on them respectively. Seems he was able to build similar graphs using EF and wanted to try for the same in LLBLGen.

EF doesn't support relationships without a PK so I doubt he managed to make it work there (in fact, EF doesn't support UCs at all)

If the relationship is properly setup with a pk-fk relationship, you can map a field onto the relationship and it's fetchable through a prefetch path though.

Thanks Frans. ( I'm ignorant when it comes to EF, I always felt it was a very poor and inefficient implementation - the sort of thing that happens when you have a few dozen devs with not much to do trying to build everything and the kitchen sink into an ORM)