Nick Craig-Wood <[EMAIL PROTECTED]> wrote:
>
> I'm adapting a legacy database for use with Django and I've come
> across a problem I haven't been able to solve!
>
> The database has ForeignKey fields which are sometimes 0, meaning
> there is no related row in the other table. If this database used
> NULL instead of 0 then I could use ForeignKey(Other, null=True) and
> all would work fine, but it doesn't and I can't change it.
>
> I've tried lots of different ways of making an adaptor to change the 0
> into a None when read from the database but I haven't made it work (or
> even get called!) yet - can anyone give me a clue?
Just in case anyone is interested, after a great deal of reading the
source code I managed to make it work. This is what I came up with
from django.db import models
from django.core.exceptions import ObjectDoesNotExist
from django.db.models.fields.related import ReverseSingleRelatedObjectDescriptor
class
OptionalReverseSingleRelatedObjectDescriptor(ReverseSingleRelatedObjectDescriptor):
"""
Part of OptionalForeignKey implementation
FIXME implement: When setting None is transated to 0
"""
def __get__(self, instance, instance_type=None):
try:
value = super(OptionalReverseSingleRelatedObjectDescriptor,
self).__get__(instance, instance_type)
except ObjectDoesNotExist:
value = None
return value
class OptionalForeignKey(models.ForeignKey):
"""
A ForeignKey, but when the related object isn't found the value is None
"""
def __init__(self, *args, **kwargs):
kwargs['null'] = True
kwargs['blank'] = True
super(OptionalForeignKey, self).__init__(*args, **kwargs)
def contribute_to_class(self, cls, name):
super(OptionalForeignKey, self).contribute_to_class(cls, name)
# Override this to get the Optional effect
setattr(cls, self.name,
OptionalReverseSingleRelatedObjectDescriptor(self))
--
Nick Craig-Wood <[EMAIL PROTECTED]> -- http://www.craig-wood.com/nick
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