take a look at this https://docs.djangoproject.com/en/3.0/topics/signals/
El lun., 27 ene. 2020 a las 18:59, Stephen J. Butler (< [email protected]>) escribió: > Frankly, if is_on_sale has such a tight constraint I wouldn't have it as > its own column. Why not just make it an annotated field with an F > expression? > > > https://docs.djangoproject.com/en/3.0/ref/models/expressions/#using-f-with-annotations > > Item.objects.annotate(is_on_sale=(F('price') < F('full_price'))) > > > > On Mon, Jan 27, 2020 at 11:47 AM Peter Law <[email protected]> wrote: > >> Hi, >> >> Thanks for adding support for check constraints in Django 2.2, it's >> great to be able to move constraints into the model definitions. >> >> I've been trying to work out how to express a constraint which >> validates that the value of one field expresses a relation between two >> other fields, but can't find a nice way to do so. >> >> I've read through the docs and also found >> https://groups.google.com/d/topic/django-users/6Olh5V1b7Us/discussion, >> but haven't found a concise spelling. >> >> I've got a model like: >> >> ``` >> class Item(Model): >> price = DecimalField() >> full_price = DecimalField() >> is_on_sale = BooleanField() >> ``` >> >> I'd like to be able to express neatly that the `is_on_sale` boolean be >> true only when `price < full_price`. >> In Postgres I can express this as: >> >> ``` >> ALTER TABLE item >> ADD CONSTRAINT is_on_sale_check >> CHECK (is_on_sale = (price < full_price)) >> ``` >> >> However in Django I can't find a way to express this directly. >> >> I did find a long spelling which essentially checks the True case and >> the False case explicitly and then ORs them together, however it's >> several lines of `models.Q` combinations and not at all clear about >> what the code is trying to achieve. >> >> Is there a concise way to do this sort of constraint? If not, would it >> be possible for Django to add support for it? >> >> Thanks, >> Peter >> >> -- >> 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 view this discussion on the web visit >> https://groups.google.com/d/msgid/django-users/CAEMtty7rsf2qk_0qXnzqZdA89e6VRCqF%2B4nQSHgxVCE4XfKAnw%40mail.gmail.com >> . >> > -- > 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 view this discussion on the web visit > https://groups.google.com/d/msgid/django-users/CAD4ANxWtpqnDr_8WrQiskH7wXxBPyJ3sTWa%2BAsK1592SXWTfpw%40mail.gmail.com > <https://groups.google.com/d/msgid/django-users/CAD4ANxWtpqnDr_8WrQiskH7wXxBPyJ3sTWa%2BAsK1592SXWTfpw%40mail.gmail.com?utm_medium=email&utm_source=footer> > . > -- 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 view this discussion on the web visit https://groups.google.com/d/msgid/django-users/CAO_yRT1F09KXXAJ9e_tdGzxjpy9TBf5B6%3D-Yyzp-Swepybay5w%40mail.gmail.com.
