public function get_foreign_keys(string $col, string $table, string $db = null): array
  {
    if ( !$db ){
      $db = $this->current;
    }
    $res = [];
    $model = $this->modelize();
    foreach ( $model as $tn => $m ){
      foreach ( $m['keys'] as $k => $t ){
        if ( ($t['ref_table'] === $table) &&
          ($t['ref_column'] === $col) &&
          ($t['ref_db'] === $db) &&
          (\count($t['columns']) === 1)
        ){
          if ( !isset($res[$tn]) ){
            $res[$tn] = [$t['columns'][0]];
          }
          else{
            $res[$tn][] = $t['columns'][0];
          }
        }
      }
    }
    return $res;
  }