I have the following model:

class Announcement(models.Model):
    ...
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)
    views = models.PositiveIntegerField(default=0, editable=False)

my view:

class AnnouncementDetailView(DetailView):
    model = Announcement
    context_object_name = 'announcement'
    template_name = 'web/index.html'

    def get(self, *args, **kwargs):
        announcement = get_object_or_404(Announcement, id=kwargs['id'])
        announcement.views  = 1
        announcement.save()
        return super().get(self, *args, **kwargs)

The problems are:

1. I know about the F expression, I'll use it down below, but I want to know another ways of updating field and not updating the updated_at field.
2. I want to save the changes of model's views field but everytime I save it, it sets the new value to updated_at field. It means everytime I refresh the page it shows the current time on updated_at field, which is very bad.

I have changed my view's code to:

...
def get(self, *args, **kwargs):
    Announcement.objects.filter(id=kwargs['id']).update(views=F('views')   1)
    return super().get(self, *args, **kwargs)

Now it works fine but I have several questions. How it saves the new value of views field without calling the save method of """"Django"""""? How far can I go with it. What is the best practices of changing some fields of model without hitting the save method?

CodePudding user response：

An easy way to do this, might be to specify update_fields in the .save(…) method ^[Django-doc]:

class AnnouncementDetailView(DetailView):
    model = Announcement
    context_object_name = 'announcement'
    template_name = 'web/index.html'

    def get(self, *args, **kwargs):
        try:
            return super().get(self, *args, **kwargs)
        finally:
            object = getattr(self, 'object', None)
            if object:
                object.views = F('views')   1
                object.save(update_fields=('views',))

It is however better to work with an F-expression to avoid race-conditions.

Making use of:

Announcement.objects.filter(id=kwargs['id']).update(views=F('views')   1)

will let the database make a query that looks like:

UPDATE appname_announcement
SET view = view   1
WHERE id = id

If you thus do not have to load the object in memory, this will be more efficient, since it makes a single roundtrip to the database.