Re: ForeignKey on CharField and char operations

[Tomas Neme]

I might not be understanding this fully, but what about
A.objects.filter(type__mnemo__startswith="type1")?

On Thu, Jul 5, 2012 at 7:51 AM, Serg Shtripling <ser...@shtripling.com> wrote:
> Hello, community!
> Have anyone tried this:
> class AType(models.Model):
> #cut
> mnemo = models.CharField(u'Mnemocode', max_length=31, null=True,
> unique=True)
>
> class A(models.Model):
> #cut
> type = models.ForeignKey(AType, 'mnemo', verbose_name=u'A Type',
> null=True)
>
> class B(models.Model):
> #cut
> type_mnemo = models.CharField(u'A Type', max_length=31, null=True,
> blank=True)
> Mnemos = {type1, type1__plus, type1__minus, type2, type2__plus,....}
> Now I want to get all A records, having AType starts with "type1", eg:
> "type1,type1__plus, type1__minus"
>
> r = A.objects.filter(type_id__startswith='type1')
> And got:
> TypeError: Related Field has invalid lookup: startswith
> As a workaround, I've replaced A model with B model - I do not have a
> relation, but query works as expected.
> Q: 1)Is it OK, that A.type_id, not just A.type field is a Related Field?
> q=AType.objects.all()[0]
>>>> type(q.type)
> <class 'main.models.AType'>
>>>> type(q.type_id)
> <type 'unicode'>
> 2) Is there a more correct way to solve this issue?
>
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[Serg Shtripling]

четверг, 5 июля 2012 г., 20:40:00 UTC+7 пользователь Tomas Neme написал:

I might not be understanding this fully, but what about
A.objects.filter(type__mnemo__startswith="type1")?

Wouldn't it create INNER JOIN query on a_type table? AFAIR it would and it's a bit overhead.
But surely it is a better solution than mine one, thanks for that.

[Tomas Neme]

> Wouldn't it create INNER JOIN query on a_type table? AFAIR it would and it's
> a bit overhead.
> But surely it is a better solution than mine one, thanks for that.

Well, in that case, I think your problem is that your AType PK isn't
the mnemo field, but some integer field, so type_id is integer, not
string.

Try defining it as

mnemo = models.CharField(u'Mnemocode', max_length=31, null=True,

unique=True, primary_key=True)

but I'm not sure if this is the behavior you want