Ah, I think I see what you mean. I don’t think you can aggregate by a field from a different data type (at least that’s how I understood your question). You can only aggregate based on the data type you are searching for.
What you might want to do (if this is your case) is create satellite data types in bookingUser where you will hold the sum. You can update that field via Database Triggers every time there is a new booking and/or there is a change to a booking.
Petter Amlie has a blog post on satellite data types you might want to check out.
Apologies, if I misunderstood your question 