[Product-Developers] how to prevent portal_catalog from indexing acquisition values?

[Kees Hink]

I have a field 'performance_code', which i want to create catalog indexes for.

Problem is that objects contained in objects of my content type will acquire the
value of its parent, so catalog queries for 'getPerformance_code' will also
return brains for objects which don't actually have that field.

I tried setting 'index_method = context.aq_explicit().performance_code' in the
schema, to no effect.

What to do?