Adding dynamic fields to a serializer in django-rest-framework

Question

I have a model

    class Event(models.Model):
        random_fields...

    class Registrations(models.Model):
        user = models.ForeignKey(User...)
        event = models.ForeignKey(Event...)
        ts = models.DateTimeField(...)

Now I want to fetch the list of all the events along with whether the user who made the request is registered in that event or not (is_registered field in the ModelSerializer of Event maybe).

This should ideally happen in a single request and not too many database queries.

What is the efficient way to do this?

ruddra · Accepted Answer · 2019-01-10 05:33:38Z

1

I think you can use prefetch related for reducing DB query. When fetching registrations from event, you can use prefetch_related to fetch all registrations at once(so multiple db query is not necessary).

So, for this first you can define the model like this:

class Registrations(models.Model):
    user = models.ForeignKey(User...)
    event = models.ForeignKey(Event, related_name="registrations",..)

Then, you can define the EventSerializer like this:

class EventSerializer(serializer.ModelSerializer):
    registrations = RegistrationSerializer(many=True, read_only=True)  # <-- getting all registraions via RegistrationSerializer
    is_invited = serializer.SerializerMethodField()

    class Meta:
       model = Event
       fields = "__all__"

    def get_is_invited(self, obj):
        return obj.registrations.filter(user=self.context.get("request").user).exists()  # does not make any DB query

SerializerMethodField has been used here to fetch if user is being invited or not.

Now in the view, when calling the Event queryset, use prefetch_related.

# A minimal example

class SomeView(APIView):
    def get(self, request, **kwargs):
        qset = Event.objects.prefetch_related('registrations').all()
        serializer = EventSerializer(qset, many=True)
        return Response(serializer.data)

edited Jan 10, 2019 at 5:33

answered Jan 9, 2019 at 8:35

ruddra

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4 Comments

Rahul Vivek Over a year ago

Hi, there is a typo in the custom field method, instead of 'get_in_invited' it is 'get_is_invited'.

Saket Patel Over a year ago

This sounds good. As far as I understand, this method will load all the registrations for each event in memory. Am I right? If it does then is there a way to do this without loading all the registrations in memory?

Saket Patel Over a year ago

Also for newer versions of django, I think select_related will not work for reverse relations. Use prefetch_related instead.

ruddra Over a year ago

select_related worked for me in django 1.11. But it does not work in 2.1. There is 2 ways to do it to fetch data, one is either query database for each event or load these data in memory.

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