Lazy Loading with Doctrine2 and Symfony2 using DQL - php

Question

I have a tree structure with a parent field. Currently I am trying to get all parent nodes to display the path to the current node.
Basically I am doing a while-loop to process all nodes.
$current = $node->getParent();
while($current) {
// do something
$current = $current->getParent();
}
Using the default findById method works. Because the entity has some aggregated fields, I am using a custom repository method, to load all basic fields with one query.
public function findNodeByIdWithMeta($id) {
return $this->getEntityManager()
->createQuery('
SELECT p, a, c, cc, ca, pp FROM
TestingNestedObjectBundle:NestedObject p
JOIN p.actions a
LEFT JOIN p.children c
LEFT JOIN c.children cc
LEFT JOIN c.actions ca
LEFT JOIN p.parent pp
WHERE p.id = :id
')
->setParameter('id', $id)
->setHint(
\Doctrine\ORM\Query::HINT_CUSTOM_OUTPUT_WALKER,
'Gedmo\\Translatable\\Query\\TreeWalker\\TranslationWalker'
)
->getOneOrNullResult();
}
With that code, loading the parents fails. I only get the immediate parent (addressed by LEFT JOIN p.parent pp) but not the parents above. E.g. $node->getParent()->getParent() returns null.
Whats wrong with my code? Did I misunderstood the lazy loading thing?
Thanks a lot,
Hacksteak

Answer 1

It looks like your are using the adjacency model for storing trees in a relational database. Which in turn means, that you will need a join for every level to get all ancestors with a single query.
As you are already using the Doctrine Extension Library I recommend to have a look at the Tree component.

Answer 2

My Answer involves not using DQL and instead creating a NestedSetManager which has access to your DBAL connection so you can use SQL. I never felt like the ORM's did a good job with NestedSets query logic.
With a NestedSetManager, you can then write a bunch of clean methods and it's really simple because all these queries are well documented. See this link. Some of the method in my NestedSetManager are:
setNode();
setRoot();
loadNestedSet();
moveNodeUp();
modeNodeDown();
getRootNode();
addNodeSibling();
getNodesByDepth();
getParents();
getNodePath();
childExists();
addChildToNode();
renameNode();
deleteNode();
// And many more
You can have a ball and create a lot of create NestedSet functionality if you're not tied down by an ORM's somewhat complex functionality.
Also -- Symfony2 makes all this really really easy. You create your NestedSetManager class file and reference it in your Services.yml and pass in your Dbal connection. Mine looks like this:
services:
manager.nestedset:
class: Acme\CoreBundle\Manager\NestedSetManager
arguments: [ #database_connection ]
you can then access your nestedsets with:
$path = $this->get('manager.nestedset')->setNode(4)->getNodePath(); // in your controller
Moral of the story, ORM/NestedSets drove me bonkers and this solution work really well. If you're being forced to use DQL and have no other options, this answer probably wont be acceptable.