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 [email protected]. To post to this group, send email to [email protected]. 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.
