Should DecimalField on postgresql adapt its values?

[Julien Palard]

Currently the DecimalField values are not adapted:

     def adapt_decimalfield_value(self, value, max_digits=None, decimal_places=None):
        """
        Transform a decimal.Decimal value to an object compatible with what is
        expected by the backend driver for decimal (numeric) columns.
        """
        return value

Say I use:

    class Doh(models.Model):
        cost = models.DecimalField("cost", max_digits=9, decimal_places=3)

Then it behave surprisingly at request time:

$ ./manage.py shell
>>> from decimal import Decimal
>>> kwargs = {"cost": Decimal("12345.123456789")}
>>> Doh.objects.create(**kwargs)
<Doh: Doh object (1)>
>>> Doh.objects.get(**kwargs)
Traceback (most recent call last):
[...]
app.models.Doh.DoesNotExist: Doh matching query does not exist.

So currently I'm using:

        with localcontext() as ctx:
            ctx.rounding = ROUND_UP
            kwargs = {"cost": round(Decimal("12345.123456789"), 3)}

But it feels fragile: what if I change the scale on the model side? I'll have to change all my calls to round.

Also it feels like implementing something that could be done in the adapter: if we know the decimal has only 3 places, allowing a request to touch the database with a WHERE with a value with more prevision will always result in no answer from the DB so there’s no point? Or am I missing something?