Hi Jaclyn,
I see that you have a File entity and a PO entity that is a generalization from File. So you’re dealing with some sort of self-reference here. Of course I'm not sure what the requirements are but first and foremost I'd reconsider this setup. It's quite a complicated thought model. Does PO need to generalize from File? I'd try removing the generalization from PO and go from there.
If PO does need to contain a FileDocument, I'd make both File and PO a generalization of System.FileDocument.
Here is my File Entity:
Here is the PO entity again:
Also, here is my comment entity that IS working correctly with association to the PO
Here you go Marius
Current Domain Model:
FIle Entity:
PO Entity: