Django REST Framework - optimizing nested serialization of a queryset

Question

I have a model and serializer like this:

models.py

class CalendarEvent(BaseMixin):
    title = models.TextField(blank=True, null=True)

class CalendarEventReminder(BaseMixin):
     event = models.ForeignKey(CalendarEvent, related_name = 'reminders')
     minutes = models.CharField()

class Meta:
    managed = False
    db_table = 'calendar_event_reminder'

def __str__(self):
    return self.minutes

serializer.py

class CalendarEventSerializer(serializers.ModelSerializer):
    reminders = serializers.StringRelatedField(many=True)

    class Meta:
        model = CalendarEvent
        fields = ('title', 'reminders')

In my view, I do the following:

def test(request):
    #...
    event = CalendarEvent.objects.filter(id__in = [930, 935])
    serializer = CalendarEventSerializer(event, many = True)
    print (serializer.data)
    #...

When I open the Debug Toolbar, I see that the database hits reminders table twice for each of the calendar events.

The question is, how this behavior could be optimized.

olieidel · Accepted Answer · 2017-09-19 00:16:05Z

2

The most straight-forward way would be prefetching the CalendarEventReminders of the CalendarEvents in your view:

# views.py
def get(request):
    event = CalendarEvent.objects.filter(id__in = [930, 935]) \
                                 .prefetch_related('reminders')
    # ...

This will prefetch all CalendarEventReminders while getting the CalendarEvents.

Note that this will not trigger a sql join like select_related() would do. We can't use select_related() in this case because we're following the relation backwards. :)

Check out the Django Docs regarding prefetch_related.

answered Sep 19, 2017 at 0:16

olieidel

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2 Comments

Edgar Navasardyan Over a year ago

Well, I know about prefetch_related. The question was, how can I do it via nested serializers. Are they intented to only serialize objects and not querysets ?

olieidel Over a year ago

Yes, serializers only serialize objects. The handling of your queryset is done in your view. Your queries as seen in the debug toolbar are triggered by simple __getitem__ calls to your queryset, that's why prefetch_related will eliminate those queries or rather, fetch them all at once at the beginning.

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