Wrong use of aggregation module may lead to non-deterministic bugs

[AmirHabibi]

Assuming that an AJAX query posts (start, limit) for retrieving Polls (Django tutorial), writing the following code leads to request.POST problem.

from django.core import serializers
from django.http import HttpResponse
from django.db.models import Count
from polls import Poll

def respond(rows, totalCount):

data = serializers.serialize('json', rows)
response = "{'success': true, 'totalCount':%d, 'rows':%s }" % (totalCount, data)
return HttpResponse(response, mimetype="application/javascript")

def listPolls(request):

start = request.POSTstart
limit = request.POSTlimit

rows = Vehicle.objects.all()[start:start+limit]
totalCount = Vehicle.objects.aggregate(Count('id'))

return respond(rows, totalCount)

surprisingly, fixing the code to:

totalCount = Vehicle.objects.aggregate(Count('id'))id__count

fixes the request.POST exception. The issue is that the django error report about request.POST not having a key named 'start' or 'limit' distracts the programmer to debugging the front end AJAX code rather than focusing on the Python bug. It also may indicate a more fundamental issue that exhibits random behavior.