[TYPO3-mvc] how to add/remove related object in an m:n relation?

[Roland]

hi everybody,

there is a class in my domain model "Item" which has a m:n relation to 
the class "Category".

how can i add/remove categories for a specific item via extbase in the 
frontend?

i added 2 new actions in the ItemController.php called "addCategory" and 
"removeCategory" but don't know which methods (of the repository?) make 
it possible to add/remove a specific categroy for this specific item.

can anybody help? thanks.

kind regards.

roland