How do update last message date in Django model

Question

I have two models where users create a post and comment on that post. When users comment this post I want updating last_message date on UserPost model. If there are no comments yet I want the default last_message date to be set to the date the post was created.

Models:

class UserPosts(models.Model):
   postTitle = models.CharField(max_length=100, verbose_name="Title")
   postContent = RichTextField(null=True, verbose_name="Content")
   created_at = models.DateTimeField(
                auto_now_add=True, verbose_name="Date of upload")
   last_message = ?????????????????

   def __str__(self):
       return self.postTitle



class UserMessages(models.Model):
    postMessages = RichTextField(null=True, verbose_name="Message")
    post = models.ForeignKey(
             UserPosts, on_delete=models.CASCADE, verbose_name="Linked Post", null=True)
   created_at = models.DateTimeField(auto_now_add=True, verbose_name="Date of upload")

EDİT

MY VİEWS:

def forums(request,forumname):
   forum = Category.objects.get(categoryName=forumname)
   post2 = UserPosts.objects.all().filter(category=forum.id).order_by("- 
                     usermessages").filter(category=forum.id)
   context = {
      'posts': post2,
   }
   return render(request, "_forums.html",context)

MY TEMPLATE:

    {% for post in posts %}
       <div class="mb-2 text-muted">
           {{post.last_message | naturaltime}}
       </div>
    {% endfor %}

I couldn't find anything relevant to this topic in the Django docs, Google, and other sources.

Answer 1

If it is an option to not have it at the database, you can just calculate this at the time when you need it. So for that you can use a model property:

class UserPosts(models.Model):
    postTitle = models.CharField(max_length=100, verbose_name="Title")
    postContent = RichTextField(null=True, verbose_name="Content")
    created_at = models.DateTimeField(auto_now_add=True, verbose_name="Date of upload")
    
    @property
    def last_message(self):
        last_message = self.usermessages_set.order_by('-created_at').first()
        if last_message:
            return last_message.created_at
        return self.created_at

So if a post doesn't have a message yet, it will get it's own created_at field. You can then use this like a property:

post = UserPosts.objects.first()
print(post.last_message)

Also consider using cached_property if you plan to use this field in a view multiple times.