code: public function getPrimaryChild($parentObject)
      {
          if (!$this->handlePrimary) {
              return false;
          }
          if (!$this->child->getPrimaryAsChild($this->parent)) {
              return false;
          }
          $key = json_encode([__FUNCTION__, $this->systemId, $parentObject->primaryKey]);
          if (!isset(self::$_cache[$key])) {
              self::$_cache[$key] = null;
              $relationClass = Yii::$app->classes['Relation'];
              $childClass = $this->child->primaryModel;
              $relation = $relationClass::find();
              $alias = $relationClass::tableName();
              $relation->andWhere(['`' . $alias . '`.`parent_object_id`' => $parentObject->primaryKey, '`' . $alias . '`.`primary_child`' => 1]);
              $relation->andWhere(['or', '`' . $alias . '`.`child_object_id` LIKE :prefix']); //, '`'. $alias.'`.`child_object_id` LIKE \''.$childClass.'\''
              $relation->params[':prefix'] = $childClass::modelPrefix() . '-%';
              $parentObject->addActiveConditions($relation, $alias);
              $relation = $relation->one();
              if (!empty($relation)) {
                  self::$_cache[$key] = $relation;
              }
          }
  
          return self::$_cache[$key];
      }
docs: Get primary child.
  
  @param [[@doctodo param_type:parentObject]] $parentObject [[@doctodo param_description:parentObject]]
  
  @return [[@doctodo return_type:getPrimaryChild]] [[@doctodo return_description:getPrimaryChild]]