How to order a Django Rest Framework ManyToMany related field?

Question

I have a Django Rest Framework application with the following (simplified) models.py:

class Photo(models.Model):
    ...

class Album(models.Model):
    ...
    photos = models.ManyToManyField(Photo, through='PhotoInAlbum', related_name='albums')

class PhotoInAlbum(models.Model):

    photo = models.ForeignKey(Photo)
    album = models.ForeignKey(Album)

    order = models.IntegerField()

    class Meta:
        ordering = ['album', 'order']

And in my serializers.py, I have the following:

class AlbumSerializer(serializers.ModelSerializer):

    ...
    photos = serializers.PrimaryKeyRelatedField('photos', many=True)

My question is, how can I have AlbumSerializer return the photos ordered by the field order?

Answer 1

The best solution to customise the queryset is using serializers.SerializerMethodField, but what shezi's reply is not exactly right. You need to return serializer.data from SerializerMethodField. So the solution should be like this:

class PhotoInAlbumSerializer(serialisers.ModelSerializer):
    class Meta:
        model = PhotoInAlbum


class AlbumSerializer(serializers.ModelSerializer):
    # ...

    photos = serializers.SerializerMethodField('get_photos_list')

    def get_photos_list(self, instance):
        photos = PhotoInAlbum.objects\
            .filter(album_id=instance.id)\
            .order_by('order')\
            .values_list('photo_id', flat=True)

        return PhotoInAlbumSerializer(photos, many=True, context=self.context).data

Answer 2

It looks as if the RelatedManager that handles the relationship for ManyToManyFields does not respect ordering on the through model.

Since you cannot easily add an ordering parameter to the serializer field, the easiest way to achieve ordering is by using a serializer method:

class AlbumSerializer(serializers.modelSerializer):
    # ...

    photos = serializers.SerializerMethodField('get_photos_list')

    def get_photos_list(self, instance):
        return PhotoInAlbum.objects\
            .filter(album_id=instance.id)\
            .order_by('order')\
            .values_list('photo_id', flat=True)