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

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 <peter@thread.com> 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

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