code: public function getUnmappedForeignKeys()
      {
          $mappedForeign = ArrayHelper::getColumn($this->_map, 'foreignKey');
          $u = array_diff(array_keys($this->foreignModel->meta->schema->columns), $mappedForeign);
          unset($u[$this->foreignPrimaryKeyName]);
  
          return $u;
      }
docs: Get unmapped foreign keys.
  
  @return array array without the mapped foreign keys