CODEWITHSUNDEEP
pythondjango
David Dahan
David Dahan

Reputation: 11152

Are Django querysets really lazy when using Paginator?

I wrote a view with pagination and multiple querysets. How ever, as I'm using namedtupled, sorted() and chain() method before actually using pagination, I was asking myself if my queries could be interpreted before I actually use Paginator.

In that case, my code would be dummy: pagination would seemed to work as expected but performances would be awful. (I can't check it now since I have a powerful computer and few data).

Could you tell me in that case if everything is ok and queries are NOT executed before pagination occurs?

scan_evts = [ScanEventTuple(
    evt_id=e.pk,
    date=e.created) \
    for e in ScanEvent.get_scan_events(shop=shop_to_check) \
             .order_by('-created')]

userewards_evts = [UseRewardEventTuple(
    evt_id=e.pk,
    date=e.created) \
    for e in UseRewardEvent.get_usereward_events(shop=shop_to_check) \
             .order_by('-created')]

redeemdeals_evts = [RedeemDealsEventTuple(
    dr_id=e.pk,
    date=e.redeem_date) \
    for e in DealRecipients.get_redeem_events(shop=shop_to_check) \
             .order_by('-redeem_date')]

event_list = sorted(
    chain(scan_evts, userewards_evts, redeemdeals_evts),
    key=attrgetter('date'),
    reverse=True)

# Pagination
paginator = Paginator(event_list, 20)
page = request.GET.get('page')
try:
    all_events = paginator.page(page)
except PageNotAnInteger: # Mauvaise parametre
    all_events = paginator.page(1)
except EmptyPage: # Page out of range
    all_events = paginator.page(paginator.num_pages)

Upvotes: 0

Views: 172

Answers (1)

Peter DeGlopper
Peter DeGlopper

Reputation: 37319

Your query sets are not lazily evaluated here. As soon as you iterate over them (as for example in for e in ScanEvent.get...), the full query set is evaluated. See:

https://docs.djangoproject.com/en/1.8/ref/models/querysets/#when-querysets-are-evaluated

I do not think you're going to be able to paginate over three different query sets without evaluating them.

Upvotes: 3

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