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!"
        }
    },

You can get the last message like this:

def get_lastmessage(self, obj):
    lastmsg = Message.objects.latest('sent_at')
    data = {'senderusername': obj.senderusername,
        'reciusername': obj.reciusername,
        'body': lastmsg.body
        'sent_at': lastmsg.sent_at
        'last_message': {
            'body': lastmsg.body
        }
    }
  
    return data