Symfony2 and Doctrine - Error: Invalid PathExpression. Must be a StateFieldPathExpression - symfony

Question

I have an entity that looks like this:
/**
* #Gedmo\Tree(type="nested")
* #ORM\Table(name="categories")
* #ORM\Entity()
*/
class Category extends BaseCategory
{
/**
* #ORM\OneToMany(targetEntity="Category", mappedBy="parent")
*/
protected $children;
/**
* #Gedmo\TreeParent
* #ORM\ManyToOne(targetEntity="Category", inversedBy="children")
* #ORM\JoinColumn(name="parent_id", referencedColumnName="id", onDelete="SET NULL")
*/
protected $parent;
}
and I am trying to run a query like this:
$qb = $this->em->createQueryBuilder()
->select('c.parent')
->from('Category', 'c');
$result = $qb->getQuery()->getArrayResult();
However, I am getting the following error:
[Semantical Error] ... Error: Invalid PathExpression. Must be a StateFieldPathExpression.
How can I select the parent_id field from my table. I have tried a bunch of variations and even if I do something like this:
$qb = $this->em->createQueryBuilder()
->select('c')
->from('Category', 'c');
I get all fields in the table except for the parent_id. This seems like Doctrine is getting in the way. How can I query for this parent_id field? or better yet how can I get all fields in the table including the parent_id

Answer 1

You can use the currently undocumented IDENTITY function to select the FK IDs in a query:
SELECT IDENTITY(c.parent) ...

Answer 2

Solution using createQueryBuilder:
$query->SELECT('pa.id')
->from('Category', 'ca');
$query->join('ca.parent', 'pa');
$result = $query->getQuery()->getArrayResult();

Answer 3

You are selecting an object that is not joined.
Like said in another answer, you have to do something like :
qb->innerJoin("c.parent", "p")

Answer 4

You can change it like this:
->select(array('i.id','identity(i.parent) parent','i.nom'))