where_not_in (Solved)

[eluser]Maglok[/eluser]
Lets say I have a array of objects that I got by using the active record class.
I have a second array of objects.

Is there anyway that I can substract one from the other? (Array1 - array2)

Example: I can easily get all open events and all the events someone signed up for. I want to also list those a person did not sign up for (yet) though.

Kind of hard to explain really. I cannot phantom a query that would let me select just the ones a person did not sign up for (since the database only records those that he DID sign up for).