[TYPO3-mvc] Filter collection

[Bernhard Marx]

Hello,

in short words:
Own Extension with 1:n-relation build through extension-builder.

e.g. Category => has many events...

Now each event has several properties e.g. beginning date...

Now I want to show a list of categories and a count how many events are related... 

=> adding a property in Category class whoch return the count of the objectcolletion.

Now I need a property / filter to count only events in future (Event.beginning >= DateTime('NOW')...

how to realize... is the any helping function or do I have loop "manually" through the events ?

Thanks in advance

Bernhard

P.S: If source-code is needed i'll provide