Symfony2 Doctrine query

Hi stackoverflow friends

I'm new in symfony2 , I don't know how to write a below query in symfony2 using createQuery()

select * from Post inner join Category on Post.category_id=Category.id inner join Priority on Post.priority_id=Priority.id order by priority_number desc

I used repository class,in which ,wrote a function

public function findAllOrderedByPriorityPost()

    {



        return $this->getEntityManager()

                ->createQuery('select p,c,pr from RodasysfourmBundle:Post p inner join 

RodasysfourmBundle:Category c  inner join RodasysfourmBundle:Priority pr order by pr.priorityNumber desc')

                ->getResult();

    }

when I used this function,I got the below error

[Semantical Error] line 0, col 85 near 'c inner join': Error: Identification Variable RodasysfourmBundle:Category used in join path expression but was not defined before.

Also which method is best using this query in a custom repository or as a service?

any help appreciated.

Source: Tips4all, CCNA FINAL EXAM