[TYPO3-mvc] How to filter output when n:1 relation between objects?

[Roland]

In my domain model there is an object "Product" and an object "Country" 
with are related n:1 (!).

How i try to output all Products which are sold in a specific Country.

The uid of the Country is present as an get parameter.

QUESTION: Where and how do i have to define the query?

i guess it is the ProductRepository.php, isn't it?