Thanks Max. In an ideal world left_connector, right_connector would
be the way to go. However, I need to account for a cable that, at one
or both ends is split into separate conductors, that go to separate
connectors (thus breaking the rule I had specified.) Nodes, with a
little embellishment, allow me to achieve this. I will investigate
your itertools suggestion.
Mike
On Oct 20, 8:38 am, Max Battcher <m...@worldmaker.net> wrote:
> adelaide_mike wrote:
> > Hi
> > My models are, essentially :
> > class Cable(models.Model):
> > cable = models.CharField(max_length=8)
>
> > class Connector(models.Model):
> > connector = models.CharField(max_length=8)
>
> > class Node(models.Model):
> > cable = models.ForeignKey(Cable)
> > connector = models ForeignKey(Connector)
>
> > So, a real world cable can be plugged in to no, one, or two connectors
> > (one at each end). The Node table describes the junctions between
> > cables and connectors.
>
> > Can I, without resorting to raw SQL, build a queryset that shows
> > connector-cable-connector?
>
> You would have a hard time even accomplishing that in raw SQL with those
> models... If you have the flexibility to change your models I would try
> something like:
>
> class Connector(models.Model):
> name = models.CharField(max_length=8)
>
> class Cable(models.Model):
> name = models.CharField(max_length-8)
> left_connector = models.ForeignKey(Connector, blank=True, null=True)
> right_connector = models.ForeignKey(Connector, blank=True, null=True)
>
> This makes your 0, 1, or 2 relationship explicit. I went with the
> left/right distinction as it is an easy "handed" way of describing them,
> but other choices may be more appropriate depending on your usage
> patterns...
>
> Otherwise, your best bet would seem to me to be to use
> Node.objects.all().orderby('cable') and group the
> connector-cable-connector by yourself. Something that I generally find
> useful in situations like that is Python's groupby function from itertools.
>
> --
> --Max Battcher--http://worldmaker.net
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