[TYPO3-mvc] MM Relation in matching()

[Sebastian Fischer]

Hi Guys,

my current problem is, that i want to select videos by given category. 
No i don't see how to get a constraint that returns me the wished records.

So structure is like video, mmtable, category. In TCA everything is fine 
and i can choose the category in the video. But i dont see who to set 
the matching.

Anyone an idea?

Greetings
Sebastian