code: public function getModel($parentObjectId, $childObjectId, $activeOnly = true)
      {
          if (is_object($parentObjectId)) {
              $parentObjectId = $parentObjectId->primaryKey;
          }
          if (is_object($childObjectId)) {
              $childObjectId = $childObjectId->primaryKey;
          }
          $key = json_encode([__FUNCTION__, $this->systemId, $parentObjectId, $activeOnly]);
          if (!isset(self::$_cache[$key])) {
              $relationClass = Yii::$app->classes['Relation'];
              $all = $relationClass::find();
              $all->where(
                  ['or', 'parent_object_id=:parentObjectId', 'child_object_id=:childObjectId']
              );
              $all->params[':parentObjectId'] = $parentObjectId;
              $all->params[':childObjectId'] = $childObjectId;
              if ($activeOnly) {
                  Relatable::doAddActiveConditions($all, false);
              }
              $all = $all->all();
              foreach ($all as $relation) {
                  $subkey = json_encode([__FUNCTION__, $this->systemId, $relation->parent_object_id, $activeOnly]);
                  if (!isset(self::$_cache[$subkey])) {
                      self::$_cache[$subkey] = [];
                  }
                  self::$_cache[$subkey][$relation->child_object_id] = $relation;
              }
          }
          if (isset(self::$_cache[$key]) && isset(self::$_cache[$key][$childObjectId])) {
              return self::$_cache[$key][$childObjectId];
          }
  
          return false;
      }
docs: Get model.
  
  @param [[@doctodo param_type:parentObjectId]] $parentObjectId [[@doctodo param_description:parentObjectId]]
  @param [[@doctodo param_type:childObjectId]]  $childObjectId  [[@doctodo param_description:childObjectId]]
  @param boolean                                $activeOnly     [[@doctodo param_description:activeOnly]] [optional]
  
  @return [[@doctodo return_type:getModel]] [[@doctodo return_description:getModel]]