On Aug 8, 8:25 pm, BenW <benwil...@gmail.com> wrote:
> Hello,
>
> I am working with a legacy database that stores IP addresses as 32bit
> packed integers. Is there a way I can use 'managed' attributes with
> filter() and exclude() etc? Please see example below:
>
> Thanks!
>
> class Host(models.Model):
>
> id = models.AutoField(primary_key=True, db_column='id')
> _ipaddr = models.PositiveIntegerField(db_column='ip_address')
>
> def _get_ipaddr(self):
> ''' Convert 32bit packed IP to string quad
> '''
> return socket.inet_ntoa(struct.pack('L', self._ipaddr))
>
> def _set_ipaddr(self, ip):
> ''' Convert string quad IP to 32bit packed
> '''
> self._ipaddr = struct.unpack('L', socket.inet_aton(ip) )[0]
>
> ipaddr = property(_get_ipaddr, _set_ipaddr)
>
> Creating new Host objects with a string IP works:
>
> >> host = Host.objects.create(ipaddr='127.0.0.1')
>
> Getting the string IP from an existing Host object works:
>
> >> host.ipaddr
> '127.0.0.1'
>
> Filtering doesn't work:>> Host.objects.filter(ipaddr='127.0.0.1').count()
>
> FieldError: Cannot resolve keyword 'ipaddr' into field. Choices are:
> id, _ipaddr
>
>
>
> This is because filter() doesn't check for the attribute 'ipaddr'
> because it's not in _meta.local_fields or wherever
> I'd really like to be able to use 'managed' attributes for querying.
> Any way to do this? I understand filter() has no way of knowing to
> first convert '127.0.0.1' to packed int before appying to _ipaddr in
> the current scenario, but I thought maybe there's a special field or
> kwarg I'm missing.
Rather than use getters and setters on a standard field, you'd be
better off creating a custom field subclass. If you do that you can
define a get_db_prep_lookup method which converts the lookup value to
the value needed to search the database. See the documentation here:
http://docs.djangoproject.com/en/dev/howto/custom-model-fields/
- and remember to set the field's __metaclass__ to SubfieldBase,
otherwise the custom methods aren't used.
--
DR.
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