Hi, You can use Django annotation feature which will add a field to your queryset objects and that field can behve like any real field
On Thu, Jun 6, 2019 at 8:22 PM Chetan Ganji <[email protected]> wrote: > Yes Olivier, You are right :P > > If I understand the problem correctly this time, you want the field > available on django model, but does not want the python to do the > calculation. > You want the database to compute the value. > > If yes, you could use a row level trigger i.e after create and after > update trigger. PostgreSQL supports it. > > > class Sales(models.Model): > buying_price = models.FloatField() > selling_price = models.FloatField() > profit = models.models.FloatField(null=True, blank=True) > > > *For your reference - * > *RE: SQL Triggers* > http://www.postgresqltutorial.com/introduction-postgresql-trigger/ > > *RE: database-side computed fields* > Instead of concatenation as in the below example, you would do some other > operation and/or aggregation which would be calculated by the database. > > https://dba.stackexchange.com/questions/21897/set-a-columns-default-value-to-the-concatenation-of-two-other-columns-values > > > If it works, as it would be on database level, you have to document it in > the readme file of your project. > Otherwise, next developer on the project might not know about the trigger > and bang his head on the floor like I did earlier, lolz :P > > I hope it helps this time :P > > > Regards, > Chetan Ganji > +91-900-483-4183 > [email protected] > http://ryucoder.in > > > On Thu, Jun 6, 2019 at 6:57 PM Olivier Dalang <[email protected]> > wrote: > >> Thanks Chetan for trying to help, but please read carefully the issue, as >> you didn't understand what I'm trying to do. >> >> In the meantime, and for the record, I found this, which works for simple >> cases. https://github.com/schinckel/django-computed-field >> >> On Thu, 6 Jun 2019 at 14:57, Chetan Ganji <[email protected]> wrote: >> >>> 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 >>> [email protected] >>> http://ryucoder.in >>> >>> >>> On Thu, Jun 6, 2019 at 4:50 PM Olivier Dalang <[email protected]> >>> 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 <[email protected]> >>>> 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 >>>>> [email protected] >>>>> http://ryucoder.in >>>>> >>>>> >>>>> On Thu, Jun 6, 2019 at 3:36 PM Olivier Dalang < >>>>> [email protected]> 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 [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/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 [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/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 [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/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. >>> 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/CAMKMUjtqs1a9t7XKoDQcPxZm1CrC7eqyD7jCZSDG12-KKyd0HA%40mail.gmail.com >>> <https://groups.google.com/d/msgid/django-users/CAMKMUjtqs1a9t7XKoDQcPxZm1CrC7eqyD7jCZSDG12-KKyd0HA%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 [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/CAExk7p1ddVqskXV%3DKQXtri5cLTwJutzFeZDz5aeDanVGTcKHtQ%40mail.gmail.com >> <https://groups.google.com/d/msgid/django-users/CAExk7p1ddVqskXV%3DKQXtri5cLTwJutzFeZDz5aeDanVGTcKHtQ%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 [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/CAMKMUjsi5JE2jSp5_p_LyHm5V1P%3DBHyH6V%2BCN37HsEoK3sr5gw%40mail.gmail.com > <https://groups.google.com/d/msgid/django-users/CAMKMUjsi5JE2jSp5_p_LyHm5V1P%3DBHyH6V%2BCN37HsEoK3sr5gw%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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