How to set XPath constraint for an enum type attribute in a data grid drop down search field?

I have a data grid for an entity with some search fields. I want to filter the data grid so that only objects with an attribute (enumeration type) value equal to certain enum values are displayed. I can do this by editing the data grid, going to data source and editing the XPath constraint. This data grid contains a drop down search field on that same enum-type attribute. I also want to limit the available enum values displayed in this drop down. However, when I edit the XPath constraint of the drop down search field itself, I am unable to reference the attribute and the enum values. How can I set this constraint if the attribute is not a string, but an enum type?  
1 answers

you cannot limit the available enum values in a datagrid dropdown