Django - 2 Foreign Key fields in one model (limit choices) - django

Question

I am trying to add simple option to my admin panel. I have category, subcategory and site models. For example:
Computers (category)
- PC (subcategory)
- Notebooks (subcategory)
Health (category)
- Diet (subcategory)
- Fitness (subcategory)
Subcategory is within category. When I am adding site in my admin panel I have list of all categories and subcategories. When I choose category (Computers) in my Subcategory field I have: PC, Notebooks, Diet, Fitness (all subcategories). I don't have any idea how can I filter this only to PC and Notebooks. Any suggestions?
class Category(models.Model):
name = models.CharField(max_length=30, unique=True,
verbose_name='Nazwa kategorii')
slug = models.SlugField()
image = models.ImageField(upload_to='category_images',
verbose_name="Image",
blank=True)
description = models.TextField(default='Description',
verbose_name="Category description")
class Meta:
verbose_name_plural = "Categories"
def save(self, *args, **kwargs):
self.slug = slugify(self.name)
super(Category, self).save(*args, **kwargs)
def image_thumb(self):
if self.image:
return '<img src="/media/%s" width="40" height="40" />' % (self.image)
else:
return('')
image_thumb.short_description = 'Thumb'
image_thumb.allow_tags = True
def __str__(self):
return self.name
class SubCategory(models.Model):
category = models.ForeignKey(
'Category',
related_name='subcategory',
on_delete=models.CASCADE,
blank=True,
null=True,
)
name = models.CharField(max_length=30)
class Meta:
verbose_name_plural = "Subcategories"
def __str__(self):
return self.name
class Site(models.Model):
category = models.ForeignKey('Category')
# sub = SubCategory.objects.filter(category=category)
subcategory = models.ForeignKey('SubCategory', related_name='subcategory')
name = models.CharField(max_length=30)