How to order a Django Rest Framework ManyToMany related field? - django

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)

Answer 3

Generally, the easiest way is to do this in your AlbumView or AlbumViewSet.
You can do this by filtering - in this case you should define a get_queryset method in your AlbumViewSet.
Anyway, this is a good solution as long as you only GET the data. If you want to have POST and PUT methods working with ordering the photos, you can do it in two ways:
stay with ManyToMany relation - patch the create method in AlbumViewSet and __create_items and restore_object method in AlbumSerializer
or
replace it with something more sophisticated - use django-sortedm2m field.
Note that the second solution does not mess with AlbumViewSet (and even AlbumSerializer!) - ordering logic stays in the relation field code.