I'm struggling with how to do the following: (there is an association 1-* as a User can rent more than 1 cars) [Rencars.Cars._User=$currentUser] AND [Rencars.Cars._User.CarReturned is empty] //pseudo code The first condition is fine and identifies associated user but I need to select only records where particular field is empty.
I assume you are selecting from the Cars entity. If so, try something like this:
CarReturned = empty]
Since CarReturned is an attribute of the entity you are selecting from, there is no need to qualify it.