Ran into a bit of trouble with a model today, after a bit of a google I came across this technique which helped me solve it:

#Raise exception on object.inspect
raise Object.inspect

#Example 1
raise order.inspect

#Example 2
raise [sub_orders.count].inspect

This simply allows you display variable values as an exception. By placing a few of these throughout your troublesome code you can simulate a fully targeted trace.