如何过滤django中的外键字段？

Question

I'm working on a basic social media project ideally where user gets to join a group and post posts in it. Here's the models i have created:

    class Group(models.Model):
       name = models.CharField(max_length=255,unique=True)
       slug = models.SlugField(allow_unicode=True,unique=True) 
       description = models.TextField(blank=True,default='')
       description_html = models.TextField(editable=False,default='',blank=True)
       members = models.ManyToManyField(User,through="GroupMember")

    class GroupMember(models.Model):
       group = models.ForeignKey(Group,related_name='memberships',on_delete=models.CASCADE)
       user = models.ForeignKey(User,related_name='user_groups',on_delete=models.CASCADE)

    class Post(models.Model):
       user = models.ForeignKey(User,related_name='posts',on_delete=models.CASCADE)
       created_at = models.DateTimeField(auto_now=True)
       message = models.TextField()
       message_html = models.TextField(editable=False)
       group = 
       models.ForeignKey(Group,related_name='posts',null=True,blank=True,on_delete=models.CASCADE)

This works fine. Problem is when i create an instance of Post. This design lets me post in any/every group. But what i want is, to let the user post only in group(s) which he's/she's a member of. I don't know how to implement this. Help me out guys.

Answer 1

One quick and simple way would be to overwrite .save() and disallow adding items where user and group is not in GroupMember:

class Post(models.Model):
    ...

    def save(self, *args, **kwargs):
        if not GroupMember.objects.filter(user=user, group=group).exists():
            raise Exception()

        super().save(*args, **kwargs)

Depending on your project and code structure, you could also use a pre_save() signal to prevent writing such posts.