XPath expression for showing only associated objects in the reference selector.

I have 2 entities, “Category” and “Subcategory”. They are associated, so many Subcategories can be associated with the same Category. I have a Product entity, where I select Category and Subcategory for this product, as reference selectors. After I select a Category, in the Subcategory dropdown I want to show only those objects, which are associated with the Category that I selected in another dropdown.   I think I have to apply an XPath constrain to the SUbcategory dropdown in order to achieve it. But how the XPath expression should look like?
1 answers

Have a look here at 2.9.2, use the 'constrained by' property to configure the expected behaviour.