Reduce the number of SQL queries in a serializer with SerializerMethodField - django

Question

I need to optimize a serializer with SerializerMethodField that contains a query.
The query is used to retrieve information from another object (Event) which is linked by a primarykey to Sensor object.
class SiteSensorSerializer(serializers.ModelSerializer):
issue = serializers.SerializerMethodField()
class Meta:
model = Sensor
fields = ('id', 'label', 'view', 'issue',)
def get_issue(self, obj):
return (
Event.objects.filter(sensor=obj, date_end__isnull=True)
.order_by('-date_start')
.exists()
)
class SiteDeviceSerializer(serializers.ModelSerializer):
label = DeviceLabelSerializer()
sensors = SiteSensorSerializer(many=True, read_only=True)
class Meta:
model = Device
fields = ('id', 'name', 'label', 'sensors')
My issue is for each sensor, the query in get_issue method is executed.
How to reduce the number of query ?
My viewset:
class SiteViewSet(viewsets.ReadOnlyModelViewSet):
permission_classes = (permissions.IsAuthenticated,)
serializer_class = SiteSerializer
filter_backends = [
DjangoFilterBackend,
filters.SearchFilter,
filters.OrderingFilter,
]
filter_fields = [
"name",
]
search_fields = ["name"]
ordering = ["name"]
def get_queryset(self):
if self.request.user.is_superuser:
return (
Site.objects.all()
.prefetch_related("devices")
.prefetch_related("devices__label")
.prefetch_related("devices__sensors")
)
else:
return (
Site.objects.filter(devices__users=self.request.user)
.prefetch_related(
Prefetch(
"devices",
Device.objects.filter(users=self.request.user).select_related(
"label"
),
)
)
.distinct()
.prefetch_related(
Prefetch(
"devices__sensors",
Sensor.objects.filter(device__users=self.request.user),
)
)
.distinct()
)

Answer 1

A minor optimization would be to remove the order_by('-date_start') from get_issue() since you are calling exists() which only returns a boolean based on whether the QuerySet has more than 0 objects within it, so there is no point in sorting the QuerySet.
get_issue() would look like the following:
def get_issue(self, obj):
return (
Event.objects.filter(sensor=obj, date_end__isnull=True)
.exists()
)