mixing index facet and search facet in xc_search_facet_date_field.inc

function xc_search_facet_date_field_form(&$form_state, $facet) {
  // this is problematic, because this facet is xc_index_facet,
  // which doesn't have metadata schema at all.
  $options['select']['name'] = array_merge(
    xc_index_facet_mapping_get_facets($facet->metadata_schema, TRUE),
    xc_format_get_fields($facet->metadata_schema)
  );