Re: How to express `foo = (bar < quox)` as a constraint

Hi Simon,

Thanks! Using an ExpressionWrapper does work for this :)

I agree it might be nice to have a bit less boilerplate here. What I'd
first tried was actually:

```
CheckConstraint(check=Q(
is_on_sale=F('price') < F('full_price'),
))
```

It would be great if that could be made to work - I think that a
spelling like that would be even better than nested Q objects (even
without ExpressionWrapper).

Many thanks,

Peter

On Mon, 27 Jan 2020 at 20:24, Simon Charette <charette.s@gmail.com> wrote:
>
> I see, I think you'll need to wrap the inner Q in an ExpressionWrapper[0]
>
> That would be
>
> CheckConstraint(check=Q(is_on_sale=ExpressionWrapper(Q(price__lt=F('full_price')), output_field=models.BooleanField())))
>
> That involves a lot of boilerplate though and it ought to work without all of it so I'd submit an optimization ticket
> to allow direct usage of Q objects for boolean field lookup right hand sides
>
> Cheers,
> Simon
>
> https://docs.djangoproject.com/en/3.0/ref/models/expressions/#django.db.models.ExpressionWrapper
>
> Le lundi 27 janvier 2020 14:32:24 UTC-5, Peter Law a écrit :
>>
>> 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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