how to filter a only one field using category__icontains code example

Example 1: django filter on related field

len(Collection.objects.filter(item__flag=True).distinct())

Example 2: django 3.0 queryset examples

>>> Entry.objects.filter(
...     headline__startswith='What'
... ).exclude(
...     pub_date__gte=datetime.date.today()
... ).filter(
...     pub_date__gte=datetime.date(2005, 1, 30)
... )