If, at the same time, perhaps ExpressionNode got a mention too, that could really open up some opportunities :)
On 17 August 2013 21:17, Some Developer <someukdevelo...@gmail.com> wrote: > On 14/08/13 10:05, Marc Tamlyn wrote: > >> (Disclaimer: I didn't write any of this code, I'm just working from my >> own possibly flawed understanding) >> >> The main point of using F is not generally for performance per se, >> rather for database consistency. In the first example, the performance >> increase is negligable, but you get two significant benefits. The first >> is that you minimise the risk of race conditions by effectively issuing >> "increment" rather than "set" to the database. The second is that you >> can increase a number of rows the same way - >> Product.objects.update(number_**sold=F('number_sold') + 1). These add >> performance in that you end up doing fewer queries, but that's actually >> what you're gaining. >> >> It is worth noting that the *only* reason to set an attribute to be an F >> expression is to then update the value in the database. In fact, you >> cannot access the value after you've done this, you get a >> "django.db.models.expressions.**ExpressionNode" instance. In that sense, >> doing product.number_sold = F('number_sold') + 1 is really a long hand >> version of the update() query. >> >> As for what expressions are valid, I believe the things you suggested do >> work, but I imagine exactly what works will depend on what operations >> the database (and backend) support. Django itself isn't doing anything >> clever here, it's just providing some architecture to allow you to >> delegate functionality to the database. >> >> I agree that the main documentation for F() should reference the update >> case (IMO as an update() rather than the longhand version). >> >> Marc >> >> >> On 13 August 2013 20:22, Daniele Procida <dani...@vurt.org >> <mailto:dani...@vurt.org>> wrote: >> >> I noticed while looking for material for >> >> <https://code.djangoproject.**com/ticket/20877<https://code.djangoproject.com/ticket/20877>> >> that >> <https://docs.djangoproject.**com/en/dev/models/instances.** >> html#updating-attributes-**based-on-existing-fields/<https://docs.djangoproject.com/en/dev/models/instances.html#updating-attributes-based-on-existing-fields/> >> > >> mentions that: >> >> product.number_sold = F('number_sold') + 1 >> >> is faster than: >> >> product.number_sold += 1 >> >> though this doesn't seem to be mentioned in the database >> optimisation page. >> >> That's easy enough to address, and >> <https://docs.djangoproject.**com/en/dev/topics/db/** >> optimization.html#do-database-**work-in-the-database-rather-** >> than-in-python/<https://docs.djangoproject.com/en/dev/topics/db/optimization.html#do-database-work-in-the-database-rather-than-in-python/> >> > >> seems like a sensible place for it. >> >> However the mentions of F() that I did find raised a number of >> questions. >> >> The F() class seems to be a handy general-purpose way to refer to >> the value of a model field.. >> >> >> Firstly, it's not explained how, in expressions like: >> >> product.number_sold = F('number_sold') + 1 >> >> (from >> <https://docs.djangoproject.**com/en/dev/models/instances.** >> html#updating-attributes-**based-on-existing-fields/<https://docs.djangoproject.com/en/dev/models/instances.html#updating-attributes-based-on-existing-fields/> >> >) >> Django knows that F('number_sold') is refers to the product model. >> >> Does it know because product.number_sold is the field that this >> expression refers to? What would happen if we did: >> >> product.number_in_stock = F('number_in_stock') - F('number_sold) >> >> (i.e. can we make such calculations multiple other fields in one >> go?), or: >> >> product.number_to_reorder = F('number_sold) >> >> for example? What are the rules of the usage syntax of F()? >> >> Secondly, the main documentation for F() >> <https://docs.djangoproject.**com/en/dev/topics/db/queries.** >> html#query-expressions/<https://docs.djangoproject.com/en/dev/topics/db/queries.html#query-expressions/> >> > >> doesn't mention this kind of use at all: it only suggests that it >> might be useful in queries. >> >> Since this use seems to be just one of multiple uses for F(), >> shouldn't a more general description of F() belong somewhere else >> (where) instead? >> >> >> Finally, are there any other useful ways to use F() not covered by >> these two examples? >> >> >> Daniele >> > > I've always found the documentation on the usage of F() to be somewhat > substandard compared to the rest of the documentation to the point were I > rarely if ever use it. > > If the documentation could be beefed up I'm sure more people would be > happy to use it in the general case rather than just small side cases. > > > -- > You received this message because you are subscribed to the Google Groups > "Django developers" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to > django-developers+unsubscribe@**googlegroups.com<django-developers%2bunsubscr...@googlegroups.com> > . > To post to this group, send email to > django-developers@**googlegroups.com<firstname.lastname@example.org> > . > Visit this group at > http://groups.google.com/**group/django-developers<http://groups.google.com/group/django-developers> > . > For more options, visit > https://groups.google.com/**groups/opt_out<https://groups.google.com/groups/opt_out> > . > -- You received this message because you are subscribed to the Google Groups "Django developers" group. 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