Added edge case handling w/ better error message for the case where you're trying to do a sum/avg of a singular ('model') association -- i.e. presumably to aggregate info about numeric ids.. but this wouldnt make any sense.
Added edge case handling w/ better error message for the case where you're trying to do a sum/avg of a singular ('model') association -- i.e. presumably to aggregate info about numeric ids.. but this wouldnt make any sense.

balderdashy/waterline,luislobo/waterline