Get user's last message Django-postman and Rest API

I've been install Django-Postman user to user messaging package. I'm trying to get user's last message with Rest API.

You can check django-postman package on here: https://pypi.org/project/django-postman/

A part of models.py

class Message(models.Model):"""    A message between a User and another User or an AnonymousUser."""    SUBJECT_MAX_LENGTH = 120    subject = models.CharField(_("subject"), max_length=SUBJECT_MAX_LENGTH)    body = models.TextField(_("body"), blank=True)    sender = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE, related_name='sent_messages',        null=True, blank=True, verbose_name=_("sender"))    recipient = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE, related_name='received_messages',        null=True, blank=True, verbose_name=_("recipient"))    sent_at = models.DateTimeField(_("sent at"), default=now)    objects = MessageManager()

Views.py

class InboxLastMessagesViewSet(viewsets.ModelViewSet):    serializer_class = InboxLastMessagesSerializer    authentication_classes = (JSONWebTokenAuthentication,)    permission_classes = (IsAuthenticated,)    def get_queryset(self):        user = self.request.user        return Message.objects.filter(Q(sender=user) | Q(recipient=user)).order_by('sender')

Serializers.py

class InboxLastMessagesSerializer(serializers.ModelSerializer):    senderusername = serializers.CharField(source='sender.username', read_only=True)    reciusername = serializers.CharField(source='recipient.username', read_only=True)    sonmesaj = serializers.SerializerMethodField()    def get_lastmessage(self, obj):           /// I'M TRYING TO CREATE A FUNCTION ON HERE FOR GET SENDER'S LAST MESSAGE ////        lastmsg = obj.latest('sent_at')        return dict(body=lastmsg)    class Meta:        model = Message        fields = ('senderusername', 'reciusername', 'body', 'sent_at', 'lastmessage')

I want to an output like this:

{"senderusername": "user","reciusername": "user2","body": "Actually that is not last message","sent_at": "2019-01-19T23:08:54Z","lastmessage": {"body": "That's conversation's last message!"        }    }, {"senderusername": "user","reciusername": "user2","body": "I said that is not last message","sent_at": "2021-05-10T23:09:42Z","lastmessage": {"body": "That's conversation's last message!"        }    },