On Jul 19, 3:00 pm, rmschne <rmsc...@gmail.com> wrote:
[snip]
> The pointer you gave, thanks very much, explains how to set the data
> model to allow nulls, which I've done already. Further, as I
> originally mentioned, I'm not using Django to control data entry, so
> the None=True in the data model doesn't really have any impact, I
> think. Perhaps i'm wrong on that; but no matter because the data with
> null parent fields are indeed in the database.
>
> My problem, perhaps badly explained, was to extract with Django
> queryset all those records in the without parents. That's what's
> vexing me.
Hi,
I think the query you are looking for is quite simple.
>From what I understand (correct me if I have got it wrong), your
models are something like this:
class ChildModel(models.Model):
foo = models.CharField(max_length=20)
parent = models.ForeignKey(Parent, null=True,
related_name="child")
class ParentModel(models.Model):
bar = models.CharField(max_length=20)
To get children without parents, you just do:
ChildModel.objects.filter(parent__isnull=True)
The first time I read your question I was really confused (I think
because you had got so many replies that weren't answering your
question, I thought it was more complicated than it actually was), and
thought you had got your child / parent analogy the wrong way and were
actually asking how to do the query from the other side (ie actually
getting childless parents), which is a bit more complicated, but not
much, so I figured out how to do that too (if you care).
To get the childless parents you do:
childless= ParentModel.objects.filter(child__isnull=True)
Be warned though, if you want to use it to find the parents that have
children (ie aren't childless), you *don't* want to do this:
parents = ParentModel.objects.filter(child__isnull=False) # won't
give you what you expect
because if the foreign key is not one to one, then you will get the
multiplicity for want of a better word. i.e. if you have one parent
record that is a ForeignKey from two children, then that parent will
be returned twice. To eliminate these duplicates, you should do:
parents = ParentModel.objects.filter(child__isnull=False).distinct()
Hope that helps in some way (rather than confuses),
Em
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