A drawing connects to parts and assemblies through the views. You need to get the view, then get the associated part or assembly. Likewise, BOMs are associated with views, not entire drawings. To get a BOM's model, you need to find its view and then access the model.
If you add a view of a single part and the BOM of interest is attached to a view of a parent assembly, there probably won't be an association of the BOM item number. If the view is of the same assembly with all other parts invisible, the association may hold.