code: def make_catalog_query(self, context, field, value):
          
  
          # get the catalogs for the context
          catalogs = api.get_catalogs_for(context)
          # context not in any catalog?
          if not catalogs:
              logger.warn("UniqueFieldValidator: Context '{}' is not assigned"
                          "to any catalog!".format(repr(context)))
              return None
  
          # take the first catalog
          catalog = catalogs[0]
  
          # Check if the field accessor is indexed
          field_index = field.getName()
          accessor = field.getAccessor(context)
          if accessor:
              field_index = accessor.__name__
  
          # return if the field is not indexed
          if field_index not in catalog.indexes():
              return None
  
          # build a catalog query
          query = {
              "portal_type": api.get_portal_type(context),
              "path": {
                  "query": api.get_parent_path(context),
                  "depth": 1,
              }
          }
          query[field_index] = value
          logger.info("UniqueFieldValidator:Query={}".format(query))
          return query
docs: Create a catalog query for the field