Quantity fields don't work using temperature units ("°C", "K", etc.)

[SamuelJennings]

Issue

Temperature units are special in pint as they "are non-multiplicative units" (see Pint docs here). The current implementation in django-pint does not handle these properly and declaring a field with temperature units and attempting to save will raise:

pint.errors.OffsetUnitCalculusError: Ambiguous operation with offset unit (degree_Celsius). See https://pint.readthedocs.io/en/stable/user/nonmult.html for guidance.

Solution

Thankfully, I think the solution is pretty simple. In quantityfields.fields, first:

class QuantityFieldMixin(object):
    ...
    def from_db_value(self, value: Any, *args, **kwargs) -> Optional[Quantity]:
        if value is None:
            return None
    #     return self.ureg.Quantity(value * getattr(self.ureg, self.base_units))
      return self.ureg.Quantity(value, getattr(self.ureg, self.base_units))

and then also,

class QuantityFormFieldMixin(object):
    ...
    def clean(self, value):
        ...
        # val = self.ureg.Quantity(val * getattr(self.ureg, units)).to(self.base_units)
        val = self.ureg.Quantity(val, getattr(self.ureg, units)).to(self.base_units)
        self.validate(val.magnitude)
        self.run_validators(val.magnitude)
        return val

As far as I'm aware, this change shouldn't affect any other fields.

[CarliJoy]

Fix is in 0.7.3