[TYPO3-mvc] Extbase complex query - using joins

[Christian M.]

Hi,

I am trying to write a query in order to fetch people who belong to a 
specific group.

I have 4 objects:
- Person
- Participation
- Department
- Group

A Person can have as many Participations as he wants.
Each Participation is related to one Department. And every Department is 
part of a Group.

I have tried the following query:

   $query = $this->createQuery();
   $query->matching(
     $query->equals('participations.department.group_name', $groupName)
     );

   $result = $query->execute();

But it fails miserably with the following:
PHP Fatal error:  Call to a member function getParentKeyFieldName() on a 
non-object in 
/home/lesscouts/domains/test.lesscouts.be/typo3_src-4.7.5/typo3/sysext/extbase/Classes/Persistence/Storage/Typo3DbBackend.php 
on line 679

Do you know how to solve this?

Thanks for your help!