Hi Simon,
Thanks for your response.
I did try that, however unfortunately I get an error when running the
migration:
django.core.exceptions.ValidationError: ["'(AND:
<django.db.models.lookups.LessThan object at 0x7fcc49998a20>)' value must
be either True or False."]
I'm using Django 2.2 LTS though testing this in 3.0 unfortunately errors in
the same way.
I think the issue with this spelling is that a `models.Q` isn't expecting
to be passed another `models.Q` as a value. I think ideally it would want
an expression there, which a `models.F` is closest to, however using a
`models.F` relation there also doesn't work (see my response to Stephen's
suggestion).
Thanks,
Peter
On Monday, 27 January 2020 18:23:28 UTC, Simon Charette wrote:
>
> Did you try
>
> class Item(Model):
> price = DecimalField()
> full_price = DecimalField()
> is_on_sale = BooleanField()
>
> class Meta:
> constraints = [
>
> CheckConstraint(check=Q(is_on_sale=Q(price__lt=F('full_price'))))
> ]
>
> I haven't tried it myself but I would expect it to work on Django 3.0.
>
> Cheers,
> Simon
>
> Le lundi 27 janvier 2020 12:47:37 UTC-5, Peter Law a écrit :
>>
>> 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
>>
>
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