code: def _prepare_facet_field_spies(self, facets):
          
          spies = []
          for facet in facets:
              slot = self.column[facet]
              spy = xapian.ValueCountMatchSpy(slot)
              # add attribute "slot" to know which column this spy is targeting.
              spy.slot = slot
              spies.append(spy)
          return spies
docs: Returns a list of spies based on the facets
          used to count frequencies.