django post_delete（）信号处理程序不起作用

Question

I am trying to make it so that the 'num_posts' field of a blog object is decremented every time a post belonging to that blog is deleted, and incremented every time a post is created. I was able to implement the overloaded save method easily enough:

def save(self, *args, **kwargs):
    '''After saving a new post, increment the num_posts value in the
    relevant blog.'''
    super(Posts, self).save(*args, **kwargs)
    self.blog_id.num_posts += 1
    tmp = self.blog_id
    tmp.save()

But for whatever reason, the logic does not work when deleting a Posts object. I'm following best practices by using signal handlers in a handlers.py file in a signals submodule. I then import the submodule in my ready() method in my TasksConfig(Appconfig) method in apps.py

I don't seem to be getting any syntax errors, or any errors at all. The num_posts field on the relevant blog simply fails to decrement. Here is the relevant code:

From my handlers.py :

from django.db.models.signals import pre_delete
from django.dispatch import receiver
from webcomics.models import Pages, Posts, Blogs

@receiver(pre_delete, sender=Pages)
def handle_page_delete(sender, **kwargs):
    obj = kwargs['instance']

    if(obj != None):
        tmp1 = obj.prev_id
        tmp2 = obj.next_id

    if(tmp1 != None):
        tmp1.next_id = tmp2
        obj.prev_id = None

    if(tmp2 != None):
        tmp2.prev_id = tmp1
        obj.next_id = None


@receiver(pre_delete, sender=Posts)
def handle_bpost_delete(sender, **kwargs):
    obj = kwargs['instance']

    if(obj != None):
        tmp = Blogs.objects.get(pk = obj.blog_id)
        tmp.num_pages = tmp.num_pages - 1

From my apps.py :

from django.apps import AppConfig

class WebcomicsConfig(AppConfig):
    name = 'webcomics'


class TasksConfig(AppConfig):
    name = 'tasks'
    verbose_name = "Tasks"

    def ready(self):
        import binshellpress.webcomics.signals.handlers

Also, here is the full Posts object in models.py , just in case you guys can see something I'm missing:

class Posts(models.Model):
    title = models.CharField(max_length=180)
    pub_date = models.DateTimeField('date publishied', default=timezone.now)
    blog_id = models.ForeignKey('Blogs', on_delete=models.CASCADE)
    series_id = models.ForeignKey('Series', on_delete=models.SET_NULL,
                                  blank=True, null=True)

    # Note: Vanilla TinyMCE Integration seems to be working. Need to
    # modify the implementation to handle links, images, etc.
    data = HTMLField()

    def __str__(self):
        return self.title

    def save(self, *args, **kwargs):
        '''After saving a new post, increment the num_posts value in the
        relevant blog.'''
        super(Posts, self).save(*args, **kwargs)
        self.blog_id.num_posts += 1
        tmp = self.blog_id
        tmp.save()

I suspect that it is something obvious. I'm just at a loss because there doesn't seem to be any error message coming from anywhere.

Answer 1

You forgot to save the updated Blogs instance in handle_bpost_delete

def handle_bpost_delete(sender, instance, **kwargs):
    instance.blog.num_pages -= 1
    instance.blog.save()

There's a simpler way to get the count of related objects, though. You can simply let the database do the count when you need it. Quite fast, and much less error prone.

Example:

from django.db.models import Count

blogs = Blogs.objects.annotate(num_pages=Count('posts'))
print(blogs[0].num_pages) 

https://docs.djangoproject.com/en/1.11/topics/db/aggregation/#following-relationships-backwards