Writable nested serializers - to_representation() not showing unless required = True

[mangelozzi]

Following this example:
Writable nested serializers

Its seems quite surprising that if one does:

album = Album.objects.get(pk=1).prefetch_related('tracks')
AlbumSerializer.to_representation(album)

The to_representation will not include the tracks information.

Further surprisingly, by change I discovered to_representation will show the the nested serializer if one marks the field as required:

class AlbumSerializer(serializers.ModelSerializer):
    tracks = TrackSerializer(many=True, required=True)  # <-- With required not the to_representation() will print the tracks.

Seems like a bug to me, I would like to confirm it is one before raising an issue.