code: public function addRelationship($parent, $child, $options = [])
      {
          $parentRef = $this->getOne($parent);
          $childRef = $this->getOne($child);
          $relationship = Relationship::getOne($parentRef, $childRef, $options);
          $parentRef->addChild($child, $relationship);
          $childRef->addParent($parent, $relationship);
  
          return true;
      }
docs: [[@doctodo method_description:addRelationship]].
  
  @param unknown $parent
  @param unknown $child
  @param unknown $options (optional)
  
  @return unknown