I think this would solve your problem. Whatever calculations needs to be done, you can do it manually before saving the Sales instance. Preferably after checking if form.is_valid() in case of django and in the function perform_create() in case of django rest framework.
class Sales(models.Model): buying_price = models.FloatField() selling_price = models.FloatField() profit = models.models.FloatField() Cheers! Regards, Chetan Ganji +91-900-483-4183 ganji.che...@gmail.com http://ryucoder.in On Thu, Jun 6, 2019 at 4:50 PM Olivier Dalang <olivier.dal...@gmail.com> wrote: > Thanks for the answer. As said, the @property + python method wouldn't > work for my use case, as I need to be able to use the field in the queryset > to filter/order (plus my actual computed field use aggregates expressions). > > Cheers, > > Olivier > > On Thu, 6 Jun 2019 at 12:36, Chetan Ganji <ganji.che...@gmail.com> wrote: > >> I had a similar need in one of my clients project, I ended up using >> @property and a python method. >> >> AFAIK, AnnotationField is absent from django models. If you would like to >> write one, below is the material you need to refer. >> https://docs.djangoproject.com/en/2.2/howto/custom-model-fields/ >> >> >> Regards, >> Chetan Ganji >> +91-900-483-4183 >> ganji.che...@gmail.com >> http://ryucoder.in >> >> >> On Thu, Jun 6, 2019 at 3:36 PM Olivier Dalang <olivier.dal...@gmail.com> >> wrote: >> >>> Dear list, >>> >>> I was wondering whether there's a package or pattern to define >>> annotations as model fields, so that they could really be used as >>> database-side computed fields. >>> >>> Currently, I do something like this (not tested, it's a simplified case): >>> >>> class SalesManager(models.Manager): >>> def get_queryset(self): >>> return super().get_queryset().annotate( >>> profit=F("selling_price")-F("buying_price") >>> ) >>> >>> class Sales(models.Model): >>> objects = SalesManager() >>> >>> buying_price = models.FloatField() >>> selling_price = models.FloatField() >>> ... >>> >>> Ideally, I'd see something like : >>> >>> class Sales(models.Model): >>> buying_price = models.FloatField() >>> selling_price = models.FloatField() >>> profit = models.AnnotationField(F("selling_price")-F("buying_price")) >>> ... >>> >>> An AnnotationField would behave like a read-only field, excepted that it >>> wouldn't have a database column. >>> >>> This would have the following benefits : >>> - much less verbose >>> - all instance related stuff in one place >>> - it would make it possible to refer to "profit" as a model field >>> - you could use the field in Meta.ordering >>> - you could easily display in django.admin without additional methods >>> >>> Of course similar result can be achieved with @property and a python >>> method, but I really need this to be on the database side to use >>> subqueries, sorting and filtering, etc. >>> >>> Does anything like this exist ? Is it possible to emulate this behaviour >>> somehow ? >>> >>> Thanks !! >>> >>> Olivier >>> >>> >>> -- >>> 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/CAExk7p3eXvCUCxB3LgZJHTrVXzLuD%3DNqUE79a-dqtUXzekXgJg%40mail.gmail.com >>> <https://groups.google.com/d/msgid/django-users/CAExk7p3eXvCUCxB3LgZJHTrVXzLuD%3DNqUE79a-dqtUXzekXgJg%40mail.gmail.com?utm_medium=email&utm_source=footer> >>> . >>> For more options, visit https://groups.google.com/d/optout. >>> >> -- >> 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/CAMKMUjscehKiqt4Gcu3Cnm%2BHKhH8Y98RnGhR6chVZ7NS5qc-sw%40mail.gmail.com >> <https://groups.google.com/d/msgid/django-users/CAMKMUjscehKiqt4Gcu3Cnm%2BHKhH8Y98RnGhR6chVZ7NS5qc-sw%40mail.gmail.com?utm_medium=email&utm_source=footer> >> . >> For more options, visit https://groups.google.com/d/optout. >> > -- > 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/CAExk7p1PBMC7rR4iBu_80pwTBoie4NydB9gk0jKAqOqNwCGhZQ%40mail.gmail.com > <https://groups.google.com/d/msgid/django-users/CAExk7p1PBMC7rR4iBu_80pwTBoie4NydB9gk0jKAqOqNwCGhZQ%40mail.gmail.com?utm_medium=email&utm_source=footer> > . > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "Django users" group. 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