code: private function isToOneRelation(array $association)
      {
          if ($this->isOrmRelation($association)) {
              return ($association['type'] & self::ORM_TO_ONE_BITMASK) === $association['type'];
          } elseif ($this->isOdmRelation($association)) {
              return $association['reference'] && $association['type'] === self::ODM_TO_ONE_VALUE;
          }
  
          return false;
      }
docs: @param array $association
  
  @return bool