Query help

Hi people,

Given this:

class Category(models.Model):
name = models.CharField()
parent = models.ForeignKey('self', blank=True, null=True)
tree_path = models.CharField() # /<root_id>/.../<parent_id>/<id>/

class Item(models.Model):
name = models.CharField()
categories = models.ManyToManyField(Category)

I need a Category queryset, with each category annotated with a count of items which have categories whose tree_path starts with the tree_path of the current category, and with categories for which this count is 0 filtered out.

In case that wasn't clear - I need an efficient way to do this:

for category in Category.objects.all():
category.num_items = Item.objects.filter(categories__tree_path__startswith=category.tree_path).count()
if category.num_items > 0:
yield category

Suggestions?

TIA, Itai

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