thank you again Bill :0 makes perfect sense now. On Thursday, December 27, 2012 12:35:16 AM UTC, ke1g wrote: > > > > On Wed, Dec 26, 2012 at 3:50 PM, Glyn Jackson <cfsp...@gmail.com<javascript:> > > wrote: > >> ke1g, thank you for such a well written reply, clearly explained, I learn >> a lot. again thanks. >> >> The only bit I'm struggling to understand is the last part you state >> >> "The use of "thisPoints" in calculating "totalPoints" doesn't change >> "thisPoints". You can still enumerate it in the template, and get the same >> result." >> > > Manager and queryset methods like "filter" return a new queryset, which is > a python class instance holding all the things that you have said about > your query, but it hasn't touched the database as yet. When you use > something like "filter" on an existing queryset, the original information > is still in the original queryset, it has been copied as necessary in the > construction of the new queryset. > > A queryset is "evaluated" only when you try to use what's in the queryset, > such as "list(qs)" or "for r in qs" (including the template language's > "for"), or when you print it in the manage.py shell, as examples. And only > when it is evaluated, does it wind up composing SQL, send it to the > database, parse the result into model instances (and cacheing these as part > of doing requests with large numbers of results in chunks, among other > things). You really get quite a number of splendid optimizations when you > use the ORM. > > So at the time I showed applying "aggregate" to the "thisPoints" queryset, > "thisPoints" hadn't touched the database. At that time, it is specifically > NOT a list of instances. It is still a queryset class instance. And the > "aggregate" method uses what's in it, but doesn't change it. Later, in > your template, where you presumably apply a "for" template tag to it, is > when it uses what it knows to get stuff for you from the database. > > Bill >
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