Sorting the django objects based on SerializerMethodField

Question

I am trying to order the user profiles based on the timestamp of the last message in between both the users.

I am using SerializerMethodField to get the timestamp of the last message.

is there any way I can sort the data?

class UserProfileSerializer(serializers.ModelSerializer):
    lastmessage = serializers.SerializerMethodField()
    class Meta:
        model = User
        fields = ['id','lastmessage']
    def get_lastmessage(self,obj): 
        k = self.context.get('view').kwargs['sid']
        data =( Message.objects.filter(receiver=obj.id,sender=k) | Message.objects.filter(sender=obj.id,receiver=k)).order_by('-timestamp').values('message','timestamp')
        if len(data) == 0:
            return ""
        else:
            data = data.first()
            data["timestamp"] = str(data["timestamp"])
        return str(data)

My view:

   class UserChatViewSet(viewsets.ModelViewSet): 
       queryset = User.objects.all() 
       serializer_class = UserProfileSerializer

Now my views return:

[{
    "id": 4,
    "lastmessage": "{'message': 'random', 'timestamp': '2020-06-14 23:49:33.077749+00:00'}"
},
{
    "id": 5,
    "lastmessage": ""
},
{
    "id": 6,
    "lastmessage": "{'message': 'sample', 'timestamp': '2020-06-14 11:53:03.880833+00:00'}"
},
{
    "id": 7,
    "lastmessage": ""
}]

But I want it to sort based on the timestamp of last message

```class UserChatViewSet(viewsets.ModelViewSet): queryset = User.objects.all() serializer_class = UserProfileSerializer``` — saibhaskar, Jun 30 '20 at 12:23

score 1 · Answer 1 · edited Jul 01 '20 at 04:14

1

The order of your response should be handled in the view .

  from django.db.models import Subquery, OuterRef
 lm = Message.objects.filter(sender=OuterRef("id"), receiver=self.kwargs['sid']).order_by('-timestamp')   
    data = User.objects.all().annotate(
        lastmessage=Subquery(
                    lm.values('timestamp')[:1]
                )
    ).order_by('-lastmessage__timestamp')

edited Jul 01 '20 at 04:14

saibhaskar

435
6
21

answered Jun 30 '20 at 12:30

Mohit Harshan

1,916
1
18
41

This Is returning: 'Subquery' object has no attribute 'order_by' – saibhaskar Jun 30 '20 at 15:01
Now this gives: This queryset contains a reference to an outer query and may only be used in a subquery. – saibhaskar Jun 30 '20 at 15:10
Same error. and I think it should be .```order_by('-lastmessage__timestamp')``` instead of ```order_by('-last_message__timestamp')``` – saibhaskar Jun 30 '20 at 15:19
thats corrected, but error is different @saibhaskar – Mohit Harshan Jun 30 '20 at 15:20
Let us [continue this discussion in chat](https://chat.stackoverflow.com/rooms/216963/discussion-between-saibhaskar-and-mohit-harshan). – saibhaskar Jun 30 '20 at 15:20

Horatiu Jeflea · Accepted Answer · 2020-06-30T12:35:45.550

1

You can overwrite list in order to achieve this:

def list(self, request, *args, **kwargs):
    response = super().list(request, args, kwargs)
    # sort response.data['results']
    return response

Also, lastmessage can be a dict instead of a str, so it's easier to work with.

edited Jun 30 '20 at 12:35

answered Jun 30 '20 at 12:32

Horatiu Jeflea

7,256
6
38
67

This will work but as you saw in my output there are some empty values for the last message which is causing an error in sorting – saibhaskar Jun 30 '20 at 16:09

Sorting the django objects based on SerializerMethodField

2 Answers2