Please submit a pull request to fix the documentation if you can. There's an open ticket for a way to get the SQL for a terminal queryset method: https://code.djangoproject.com/ticket/18631
On Wednesday, February 8, 2017 at 5:09:25 PM UTC-5, Viktor Bale wrote: > > Ok, you spurred me on to actually try it out. And the answer is > interpretation (i) is correct. Which means the example in the cheat sheet > is somewhat misleading, since it does't calculate price per page, rather > the sum of prices per page for individual books. > > BTW, while playing with that little conundrum, I couldn't work out how to > display in the shell the SQL query the aggregate() function will execute > before actually executing it. You can find it after the act using "from > django.db import connection; print(connection.queries)". And printing > str(queryset.query) doesn't work like for annotate(), since aggregate() is > terminal and returns a dict (unlike annotate() which returns another > queryset). Any ideas? > > > > On Wednesday, 8 February 2017 20:10:59 UTC+1, Tim Graham wrote: >> >> As for me, this statement from that page applies, "It’s difficult to >> intuit how the ORM will translate complex querysets into SQL queries so >> when in doubt, inspect the SQL with str(queryset.query) and write plenty >> of tests." >> >> I'm not good at figuring out at a glance what an aggregation query will >> do. Do some experimentation and you should be able to figure it out >> yourself. >> >> On Wednesday, February 8, 2017 at 12:41:24 PM UTC-5, Viktor Bale wrote: >>> >>> Hi all >>> No takers, eh? Does this means it also confuses other more experienced >>> people, or that it's a silly question? If the aggration function was Avg >>> instead of Sum, then it would make perfect sense (the average price per >>> page over all books)... but it doesn't, so it doesn't :-) >>> Thanks >>> >>> >>> On Monday, 6 February 2017 13:24:49 UTC+1, Viktor Bale wrote: >>>> >>>> This is confusing me. >>>> >>>> In the topic guide on Aggregation ( >>>> https://docs.djangoproject.com/en/1.10/topics/db/aggregation/#cheat-sheet), >>>> >>>> there is an example in the cheat sheet as follows: >>>> >>>> # Cost per page>>> from django.db.models import F, FloatField, Sum>>> >>>> Book.objects.all().aggregate(... >>>> price_per_page=Sum(F('price')/F('pages'), >>>> output_field=FloatField())){'price_per_page': 0.4470664529184653} >>>> >>>> >>>> Does this mean: >>>> i) Calculate the price per page for each book (F('price')/F('pages')) >>>> and then aggregate these over all books using the Sum function, or >>>> ii) Aggregate the price using the Sum function, and also the number of >>>> pages using the Sum function, and them divide the former by the latter. >>>> ? >>>> >>>> Interpretation (i) is what the statement seems to say but this will >>>> result in a fairly meaningless number (the sum of cost per page per book, >>>> which is dependant on the number of books and doesn't mean much, and is >>>> certainly not an average price per page or anything similar). >>>> >>>> Interpretation (ii) would make more sense (it calculates the overall >>>> price per page over all books), but the fact that the expression >>>> (F('price')/F('pages')) is inside the Sum function suggests that this is >>>> not what is happening. >>>> >>>> Additionally, to aid my understanding, depending on which >>>> interpretation is correct, what would the statement be to implement the >>>> other interpretation? e.g. is interpretation (i) is what the above >>>> statement is doing, how would interpretation (ii) be implmented? Or vice >>>> versa.... >>>> >>>> Thanks >>>> >>>> -- You received this message because you are subscribed to the Google Groups "Django users" group. To unsubscribe from this group and stop receiving emails from it, send an email to django-users+unsubscr...@googlegroups.com. To post to this group, send email to django-users@googlegroups.com. Visit this group at https://groups.google.com/group/django-users. To view this discussion on the web visit https://groups.google.com/d/msgid/django-users/30991349-6858-4797-86af-ad9c4cac425e%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
