Is it possible to select as an object that entity with no relation in DQL?

Question

For example, there is a DQL that get some articles.

$query = $this->getEntityManager()->createQuery('
    SELECT a, t, u FROM MyArticleBundle:Article a
    LEFT JOIN a.tags t
    LEFT JOIN a.author u
    WHERE a.id IN (1,2,3)
    ORDER BY a.id DESC
');

I use this data as an object.

Such as:

$article->getId();
$article->getAuthor();
etc...

But, it is occurred that needs to be joined with no relation data to this data.
For example, I get comments with data.

$query = $this->getEntityManager()->createQuery('
    SELECT a, t, u, c FROM MyArticleBundle:Article a
    LEFT JOIN a.tags t
    LEFT JOIN a.author u
    LEFT JOIN MyCommentBundle:Comment c
        WITH c.chain_id = a.id AND c.app = :appName
    WHERE a.id IN (1,2,3)
    ORDER BY a.id DESC
')
->setParameter('appName', 'article');

It may be possible to get the data.
But, program (such a '$article->getId();') does not work.

FatalErrorException: Error: Call to a member function getId() on a non-object

A problem point is 'c' at 'SELECT a, t, u, c '.
Is it possible to select as an object that comments with no relation in DQL?
Should I correct the program.

---

EDIT:

DB(Table) relationships are not.
I wouldn't like to relate comment table to article table because I want to manage it separately from other app.
However, the connected ID exists.
It is 'chain_id' and 'app' at the case of above code.

So I could get related comments of article.
Problem is that fetching data is array.

What I mean is, I would like to get comments data as an child node of article data object at the case of above code.
I want to access the data like $article->comments.

This is the code that that I've seen before.

$this->bind(
  array('SomeLog as SomeLogs', array(
    'local'   => 'id',
    'foreign' => 'record_id',
  )),
  Doctrine_Relation::MANY // means hasMany
);

I think this code is for doctrine version 1.
I think that it is used when combining to force the table with no relation.

I hope that perhaps there is another way in doctrine version 2.