[Fixed]-Django Inner Join Queryset

A.objects.filter(b__c__isnull=False) results a sql w/ same result:

SELECT DISTINCT a.* FROM a 
INNER JOIN b ON (a.id = b.a_id)
INNER JOIN c ON (b.id=c.b_id)
WHERE c.id IS NOT NULL;

P.S. Why do you use IntegerField instead of AutoField for ids?