Django Rest Framework how to save a model with Related Field based on ID

Question

I'm kind of new to DRF. I have Record model that looks like this:

class Records(models.Model):
    owner = models.ForeignKey(User, null=True)
    activity = models.ForeignKey(Activity, null=True)
    time_start = models.DateTimeField(null=True)
    time_end = models.DateTimeField(null=True)

  ...

The RecordSerializer is this one:

class RecordSerializer(serializers.ModelSerializer):

    now = datetime.today()
    owner = serializers.Field(source='owner.username')
    time_start = serializers.DateTimeField(source='now')

    class Meta:
        model = Records
        fields = ("owner", "activity", "time_start")

And this is the view:

class StartApiView(generics.CreateAPIView):
    model = Records
    serializer_class = RecordSerializer

    def pre_save(self, obj):
        obj.owner = self.request.user

The POST request is sent from Backbone and it includes a field with the activity id, for example "{activity:12}". What should I do if I want the view to save the Record and set the activity to the Activity with the id of 12?

Answer 1

The accepted answer was true for DRF v2.x but is no longer for newer (3.x) versions, as it would raise this AssertionError:

AssertionError: Relational field must provide a queryset argument, or set read_only=True.

For newer versions, just add the queryset argument to it:

class RecordSerializer(serializers.ModelSerializer):
    activity = serializers.PrimaryKeyRelatedField(queryset=Activity.objects.all())
    // [...]