TreeTable and Hierarchy Property

I am trying to create a treeTable but stuck at selecting the Hierarchy property. In my example, I have 3 entities i.e. Vendor, PurchaseOrder, PurchaseOrderLine. The first entity will be parent of 2nd and 3rd entitis, similarly, 2nd entity will be the parent of 3rd entity. First, I am not sure how to get a hierarchy property as I see the pull-down option is empty.

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I have tried it, must have been missing something somewhere while creating the hierarchicalDatasource. The problem to create Hierarchy Property, nothing is there to select. I tried with ordertreetable screen in the attached project. Here I want to group by customers, orders and want to show contents from both order and orderline entities (of course customer, product as well)

demo-mortoza-2.7z (166.0K)