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
– saibhaskar, Commented Jun 30, 2020 at 12:23

Horatiu Jeflea · Accepted Answer · 2020-06-30 12:35:45Z

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, 2020 at 12:35

answered Jun 30, 2020 at 12:32

Horatiu Jeflea

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2 Comments

saibhaskar Over a year ago

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

Paul Maurer Over a year ago

For the "# sort response.data['results']" line, I used the following sorted_data = sorted(response.data, key=lambda k: k["last_message"]["timestamp"], reverse=True) then response.data = sorted_data return response. (Note this only works if there are no null values in timestamp.)

saibhaskar · Accepted Answer · 2020-07-01 04:14:12Z

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 1, 2020 at 4:14

saibhaskar

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answered Jun 30, 2020 at 12:30

Mohit Harshan

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5 Comments

saibhaskar Over a year ago

This Is returning: 'Subquery' object has no attribute 'order_by'

saibhaskar Over a year ago

Now this gives: This queryset contains a reference to an outer query and may only be used in a subquery.

saibhaskar Over a year ago

Same error. and I think it should be .order_by('-lastmessage__timestamp') instead of order_by('-last_message__timestamp')

Mohit Harshan Over a year ago

thats corrected, but error is different @saibhaskar

saibhaskar Over a year ago

Let us continue this discussion in chat.

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