[Python] 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!"
}
},

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