Symfony 2: INNER JOIN on non related table with doctrine query builder

I'm trying to build a query with the doctrine query builder which joins a non related table like this:

$query = $this->createQueryBuilder('gpr')
        ->select('gpr, p')
        ->innerJoin('TPost', 'p')
        ->where('gpr.contentId = p.contentId')

But this doesn't work. I still get an error:

Error: Identification Variable TPost used in join path expression but was not defined before.

I searched for this error message and everybody answered to use the table alias + attribute like p.someAttribute. But the table I want to join isn't related in the table I start my select from.

As a normal mysql query i would write it like this:

SELECT * FROM t_group_publication_rel gpr 
INNER JOIN t_post p 
WHERE gpr.content_id = p.content_id

Any ideas what i'm doing wrong?

Today I was working on similar task and remembered that I opened this issue. I don't know since which doctrine version it's working but right now you can easily join the child classes in inheritance mapping. So a query like this is working without any problem:

$query = $this->createQueryBuilder('c')
        ->select('c')
        ->leftJoin('MyBundleName:ChildOne', 'co', 'WITH', 'co.id = c.id')
        ->leftJoin('MyBundleName:ChildTwo', 'ct', 'WITH', 'ct.id = c.id')
        ->orderBy('c.createdAt', 'DESC')
        ->where('co.group = :group OR ct.group = :group')
        ->setParameter('group', $group)
        ->setMaxResults(20);

I start the query in my parent class which is using inheritance mapping. In my previous post it was a different starting point but the same issue if I remember right.

Because it was a big problem when I started this issue I think it could be also interesting for other people which don't know about it.

Joins between entities without associations were not possible until version 2.4, where you can generate an arbitrary join with the following syntax:

$query = $em->createQuery('SELECT u FROM User u JOIN Blacklist b WITH u.email = b.email');

Reference: http://docs.doctrine-project.org/projects/doctrine-orm/en/latest/reference/dql-doctrine-query-language.html