Django Rest-Framework nested serializer order

Question

Is there a way to order a nested serializer _set, for example order by pk or time-stamp.

So basically order song_set shown in the json data below from the most recent to the latest object created, in this case by order_by('-timestamp') or order_by('-pk').

Json data

{
    "pk": 151,
    "album_name": "Name",
    "song_set": [
         {
           pk: 3,
           timestamp: '5 seconds'
         },
         {
           pk: 2,
           timestamp: '10 seconds'
         },
         {
           pk: 1,
           timestamp: '15 seconds'
         }
    ]
}

Model

class Album(models.Model):
    album_name     = models.CharField(max_length=100, blank=True)


class Song(models.Model):
    album          = models.ForeignKey('album.Album', default=1)
    timestamp      = models.DateTimeField(auto_now_add=True, auto_now=False)

Serializer

class SongListSerializer(HyperlinkedModelSerializer):
    class Meta:
        model = Song
        fields = [
            'pk',
            'timestamp'
        ]

class AlbumSerializer(HyperlinkedModelSerializer):
    song_set = SongListSerializer(many=True, read_only=True)
    class Meta:
        model = Album
        fields = [
            'pk',
            'timestamp',
            'song_set'
        ]

related: stackoverflow.com/q/44418189

djvg
– djvg

2022-10-17 11:50:04 +00:00
Commented Oct 17, 2022 at 11:50 — djvg
– djvg, Commented Oct 17, 2022 at 11:50

Muhammad Hassan · Accepted Answer · 2019-06-25 10:15:57Z

77

You can use SerializerMethodField and write custom method for this.

class AlbumSerializer(HyperlinkedModelSerializer):
    song_set = serializers.SerializerMethodField()
    class Meta:
        model = Album
        fields = [
            'pk',
            'timestamp',
            'song_set'
        ]

    def get_song_set(self, instance):
        songs = instance.song_set.all().order_by('-timestamp')
        return SongListSerializer(songs, many=True).data

edited Jun 25, 2019 at 10:15

user1663023

answered Jan 14, 2018 at 13:11

Muhammad Hassan

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5 Comments

Jiss Raphel Over a year ago

How can I pass a variable to filter the values with a field?

Muhammad Hassan Over a year ago

You can pass data in serializer context.

Jiss Raphel Over a year ago

I did that way, but was thinking like If I can pass the set of filtered instances along with the data like "AlbumSerializer(data,song_instances)" this is wrong but is ther ea similar way to directly pass instances to nested list serializer without using serializermethod()?

Muhammad Hassan Over a year ago

If you do not want to filter your data on the basis of your instance, than you can pass filtered instances to you serializer. If tou want to pass data directly to your nested serialized, than that data must be part of your queryset.

Lis Over a year ago

This will cause your schema generator to be unable to determine the type of song_set field. If using libraries like drf_spectacular you will need to extend_schema_field or at least provide a type hinting for the get_song_set property.

intramuros · Accepted Answer · 2020-02-13 18:11:26Z

65

Add ordering meta parameter to your Song model:

class Song(models.Model):
    album = models.ForeignKey('album.Album', default=1)
    timestamp = models.DateTimeField(auto_now_add=True, auto_now=False)

    class Meta:
        ordering = ['timestamp', 'pk']

answered Feb 13, 2020 at 18:11

intramuros

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4 Comments

Yeonghun Over a year ago

This solved my issue when i didn't want to adopt the SerializerMethodField solution

djvg Over a year ago

Beware this will affect every use of the Song model. It's not limited to the serializer.

Gordon Wrigley Over a year ago

per what @djvg said, this is a bad idea performance wise

PawelRoman Jan 31 at 14:51

It's a very bad practice, because every time you make a query (even fetching only a few records) the DB will run a full table scan! Either create an index on the timestamp or use the other method

Endre Both · Accepted Answer · 2019-03-23 16:04:42Z

37

In your ViewSet, you can specify a queryset with a custom Prefetch object that you can filter and order as you like. Prefetching causes just one additional database query (instead of one per parent object when using SerializerMethodField), giving vastly improved performance.

from rest_framework import viewsets
from django.db.models import Prefetch

class AlbumViewSet(viewsets.ModelViewSet):
    queryset = Album.objects.prefetch_related(Prefetch('song_set',
        queryset=Song.objects.order_by('-timestamp')))

answered Mar 23, 2019 at 16:04

Endre Both

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Comments

SaturnFromTitan · Accepted Answer · 2019-01-24 22:21:42Z

8

Old thread, but because it's still popping up on Google I want to share my answer as well. Try overwriting the Serializer.to_representation method. Now you can basically do whatever you want, including customising the sorting of your response. In your case:

class AlbumSerializer(HyperlinkedModelSerializer):
    song_set = SongListSerializer(many=True, read_only=True)
    class Meta:
        model = Album
        fields = [
            'pk',
            'timestamp',
            'song_set'
        ]

    def to_representation(self, instance):
        response = super().to_representation(instance)
        response["song_set"] = sorted(response["song_set"], key=lambda x: x["timestamp"])
        return response

answered Jan 24, 2019 at 22:21

SaturnFromTitan

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2 Comments

Saad Aleem Over a year ago

I'm sure there could be a better way to do this. I think changing the order through the ModelManager could be a better approach. Not sure about it, though.

MooCow Over a year ago

This looks like it could have run time performance impacts since sorting seems to be left to the server and the not the database.

niallm90 · Accepted Answer · 2022-10-26 07:36:11Z

Another way to solve this is to modify the to_representation method on the ListSerializer to change the ordering when the relationship is queried via the ORM.

This to_representation is similar to how it works for the base ListSerializer in DRF.

class SongListSerializer(HyperlinkedModelSerializer):
    class Meta:
        model = Song
        fields = [
            'pk',
            'timestamp'
        ]

    def to_representation(self, data):
        iterable = data.all().order_by('-timestamp', '-id') \
            if isinstance(data, models.Manager) else data

        return [self.child.to_representation(item) for item in iterable]



class AlbumSerializer(HyperlinkedModelSerializer):
    song_set = SongListSerializer(many=True, read_only=True)
    class Meta:
        model = Album
        fields = [
            'pk',
            'timestamp',
            'song_set'
        ]

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