gistfile1.rb

SELECT st.id, sum(r.number_of_people) as people_total
FROM scheduled_tours as st 
LEFT JOIN reservations AS r ON st.id = r.scheduled_tour_id 
LEFT JOIN products AS p ON p.id = st.product_id 
WHERE st.date = '2012-11-08' 
GROUP BY st.id, p.max_capacity 
HAVING SUM(r.number_of_people) IS null OR SUM(r.number_of_people) < p.max_capacity;

ScheduledTour.
  joins(:reservations, :product).
  where("date = ?", '2012-11-08').
  group("scheduled_tours.id, scheduled_tours.product_id, scheduled_tours.created_at, scheduled_tours.updated_at, scheduled_tours.date, scheduled_tours.comments, scheduled_tours.cancelled, scheduled_tours.on_the_fly, scheduled_tours.product_schedule_id, scheduled_tours.latest_api_reservation, products.max_capacity").
  having("sum(reservations.number_of_people) < products.max_capacity")