Serializer's optimization

[r***@gmail.com]

Hello, can you help me with next problem:
I have next structure:


#models.py

Foo(BaseModel):
....

Bar(BaseModel):
id = models.UUIDField()
foo = models.ForeignKey(Foo)

Baz(BaseModel):
bar = models.ForeignKey(Bar, related_name='bazes')


Qux(BaseModel):
baz = models.ForeignKey(Baz, related_name='quxes')
q_type = models.IntegerField() # choiceField from ((0, daily), (1,
nightly))

class Meta:
unique_together = ('baz', 'q_type')

Mos(BaseModel):
qux = models.ForeignKey(Qux, related_name='moses')


# serialziers.py
class BazSerializer(serializers.ModelSerializer):
daily = serializers.SerizlierMethodField()
nightly = serializers.SerizlierMethodField()

class Meta:
model = Baz
fields = (..., 'daily', 'nightly')

def get_daily(self, instance):
try:
quxes = instance.quxes.filter(q_type=0).last().moses.all()
except:
return None
return QuxSerializer(quxes, many=True).data

class BarSerializer(serializers.ModelSerializer):
bazes = serializers.SerizlierMethodField()

class Meta:
model = Bar
fields = ('id', 'bazes')

def get_bazes(self, instance):
qs = instance.bazes

return BazSerializer(qs, many=True).data


# views.py
class BarListView(ListAPIView):
serializer_class = BarSerializer

def get_queryset(self):
foo_id = self.kwargs.get('foo_id')
qs = Bar.objects.filter(foo = foo_id)

return qs


When tried to prefetch data like that:

def get_queryset(self):
foo_id = self.kwargs.get('foo_id')
select_related = ['foo']
prefetch_related = ['bazes', 'bazes__quxes', 'bazes__quxes__moses']
qs = Bar.objects.select_related(*select_related).prefetch_related(*
prefetch_related).filter(foo = foo_id)

return qs


But how i can use this prefetched data in serializer, cause every

quxes = instance.quxes.filter(q_type=0).last().moses.all()
*in BazSerializer*

is one more query to database

Thanks, if you understood my question

[Oleg Nykolyn]

Hi,

Rewriting filter() and last() as operations manual over list/iterator in
"quxes = instance.quxes.filter(q_type=0).last().moses.all()" should
probably help.
Something like:
quxes = [q for q in instance.quxes.all() if q.type==0][-1].moses.all()
This should avoid extra DB queries.

---
Best regards,
Oleg