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Django annotation with nested filter
for student in Student.objects.all():
    student.num_excused_absence = Attendance.objects.filter(student=student, type="Excused").count()

# a query for students
students = Students.objects.all()
# a query to count the student attendances, grouped by type.
attendance_counts = Attendence(student__in=students).values('student', 'type').annotate(abs=Count('pk'))
# regroup that into a dictionary {student -> { type -> count }}
from itertools import groupby
attendance_s_t = dict((s, (dict(t, c) for (s, t, c) in g)) for s, g in groupby(attendance_counts, lambda (s, t, c): s))
# then use them efficiently:
for student in students:
    student.absences = attendance_s_t.get(student.pk, {}).get('Excused', 0)

excused = Student.objects.filter(attendance__type='Excused').annotate(abs=Count('attendance'))