In Django Rest Framework, how to restrict fields fetched with a RelatedField Serializer relation?

Question

Taking this example of a Serializer from the DRF docs :

class AlbumSerializer(serializers.ModelSerializer):

    tracks = serializers.HyperlinkedRelatedField(
        many=True,
        read_only=True,
        view_name='track-detail'
    )

    class Meta:
        model = Album
        fields = ('album_name', 'artist', 'tracks')

When DRF does the query to fetch all the track s for a particular album , the SQL generated selects all of each track 's columns. How would I restrict it to only request specific columns, eg the pk that would be used for the Hyperlinks to each track in the response?

(Why? Because there might be a lot of tracks returned and each one have a lot of data in its row in the database, which could be a lot of unnecessary data returned.)

Answer 1

You can use SerializerMethodField to build yourself the queryset and use value_list .

class AlbumSerializer(serializers.ModelSerializer):

    tracks = serializers.SerializerMethodField()

    def get_tracks(self, instance):
        return TracK.objects.filter(album=self).values_list('id', flat=True)


    class Meta:
        model = Album
        fields = ('album_name', 'artist', 'tracks')

Answer 2

I think this does it...

from rest_framework import serializers
from rest_framework.reverse import reverse

from myapp.models import Album


class AlbumSerializer(serializers.ModelSerializer):

    tracks = serializers.SerializerMethodField()

    class Meta:
        model = Album
        fields = ('album_name', 'artist', 'tracks')

    def get_tracks(self, instance):
        "Returns a list of URLs for Tracks' API detail views"

        request = self.context.get('request')

        tracks = []

        for track in instance.tracks.values('pk'):
            tracks.append(reverse(
                                'track-detail',
                                kwargs={'pk': track['pk']},
                                request=request
                            ))

        return tracks

We're manually creating the result of a HyperlinkedRelatedField() (returning a list of URLs to the track's API detail pages), but restricting the data fetched from the database using values() in the queryset.

This assumes that the track-detail URL pattern has a parameter of pk .

Replacing a PrimaryKeyRelatedField() is even simpler. Replace the get_tracks() method above with this:

    def get_tracks(self, instance):
        "Returns a list of Track IDs"
        return instance.tracks.values_list('pk', flat=True)

Thanks to @jmny for heading me towards using SerializerMethodField() .