Form InventOnhandItem InventDimId

In my situation I need to display a field from InventItemLocation on the InventOnhandItem grid so I created a display method on InventSum datasource, the InventItemLocation find method requires itemId AND InventDimId as parameters. I see there are both ItemId and InventDimId in InventSum datasource but InventDimId always blank. I tried to pass inventDIm.InventDimId as someone suggested using joinChild() method but no luck it is always null or empty.

Any suggestions ?