Problem with durationfield v2.0.3

[fuxb]

Hi,

I just installed thecut-durationfield with pip, getting version 2.0.3.
The durationfield is a great idea since django doesn't support postgres
relative timedelta like 'mons' or 'year'. Thanks a lot for making this!

Unfortunately I have problems using the RelativeDeltaField, using a
model like documented on
https://pypi.python.org/pypi/thecut-durationfield/2.0.3:

class MyModel(models.Model):
    duration = RelativeDeltaField(blank=True, null=True)

my_instance = MyModel(duration='P7D')
datetime(2014, 1, 1) + my_instance.duration

If I do this, I get an Error:
TypeError: unsupported operand type(s) for +: 'datetime.datetime' and 'str'

The same happens if I call the obejcts save method:
AttributeError: 'str' object has no attribute 'days' [save_traceback.txt](https://github.com/thecut/thecut-durationfield/files/421942/save_traceback.txt)

see attached save_traceback.txt

Obviously the 'P7D' string is not converted to relativedelta.

When I do this:

my_instance = MyModel(duration=relativedelta.relativedelta(days=7))
datetime(2014, 1, 1) + my_instance.duration
my_instance.save()

The above code works, also the save method, but only until I fetch the
entry from the Database:

In [15]: my_instance = MyModel.objects.all()[0]
In [16]: my_instance.duration
Out[16]: u'P7D'

Here the string is not converted back to a relativedelta object.

Any support would be highly appreciated!

Best regards,
Juergen

[mattaustin]

Thanks for the report. I can also replicate this using Django 1.8.

It does work correctly in version 1.0.8 (which was never pushed to pypi), and is incorrect in 2.0 - so it's a regression.

I suspect this could be related to the removal of SubfieldBase in commit 29f48ab. The docs at https://docs.djangoproject.com/en/1.8/howto/custom-model-fields/#converting-values-to-python-objects state:

Changed in Django 1.8:

Historically, Django provided a metaclass called SubfieldBase which always called to_python() on assignment. This did not play nicely with custom database transformations, aggregation, or values queries, so it has been replaced with from_db_value().

But nothing was modified on the field to provide from_db_value.

Commit 13a3a34 is also slightly worrying, as it claims 'some fixes' for things which aren't described / referenced / tested.

[g-as]

@mattaustin could you open a PR with your fixes?