It may help to know that QuerySet.__contains__() was implemented until Django 1.6 when chunked reads were removed from QuerySet iteration:
https://github.com/django/django/commit/70679243d1786e03557c28929f9762a119e3ac14 On Tuesday, June 2, 2020 at 7:43:25 AM UTC-4, Aymeric Augustin wrote: > > On 2 Jun 2020, at 13:30, Florian Apolloner <f.apo...@gmail.com > <javascript:>> wrote: > > On Tuesday, June 2, 2020 at 11:28:34 AM UTC+2, Adam Johnson wrote: >> >> If you already fetched the queryset, `if obj in queryset` will make a new >>> database query >> >> >> I don't see why we couldn't implement __contains__ to do a walk through >> _result_cache if it has been fetched? >> > > we are doing the same for len() IIRC. I have to say it sometimes makes it > a little bit hard to debug, but if it is documented as such it should be > okay. > > > These are good points. > > As far as I can tell, the current implementation of __len__ is to fetch > the whole queryset: > > def __len__(self): > self._fetch_all() > return len(self._result_cache) > > I'm almost sure doing an automatic .count() was discussed in the past. I > don't remember why we rejected it — perhaps because in most cases, when you > need the length of a queryset, you will also iterate it? > > The suggestion for __contains__ is similar: doing an automatic .filter(pk= > obj.pk).exists(). We could do it only if the queryset isn't fetched yet, > but I'm afraid this may be too magical. Is it reasonable to have `total = > len(queryset); found = obj in queryset` make one SQL query while `found = > obj in queryset; total = len(queryset)` makes two queries? > > Now I'm worried that we may be making inconsistent decisions between > __len__ and __contains__... Let's make sure we're consistent across dunder > methods. > > -- > Aymeric > > > -- You received this message because you are subscribed to the Google Groups "Django developers (Contributions to Django itself)" group. To unsubscribe from this group and stop receiving emails from it, send an email to django-developers+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/django-developers/55b4d7e0-ba30-48b2-8005-d27adacc7fd0%40googlegroups.com.
