limit choices in the drop down of subcategory based on related category in Django admin

[vikk]

Hi,

I have three models as

class Category(models.Model):

        name = models.CharField(max_length=128)


class SubCategory(models.Model):
        category = models.ForeignKey(Category)
        name = models.CharField(max_length = 400)

class Document(models.Model):
        category = models.ForeignKey(Category, null=True, blank=True,
help_text=_('Required'))
        subcategory = models.ForeignKey(SubCategory, null=True, blank=True,
help_text=_('Required'))
        title = models.CharField(max_length=300)

Now in Admin interface I have category, subcategory and title field.
If a user is trying to select any subcategory then only that
subcategory item should be displayed which are related to Category.
A simple example is Country, state dropdown.
I am trying to get it from Modelform like

class DocumentAdminModelForm(ModelForm):

    def __init__(self, *args, **kwargs):
            super(DocumentAdminModelForm, self).__init__(*args,
**kwargs)
            self.fields['sub_category'] =
forms.ModelChoiceField(queryset =
SubCategory.objects.filter(category__id = self.fields['category'] ))

but it is not working. Should I use some ajax and jquery for this or
is there any other way you can suggest ??

Thanks

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