使用保存方法扩展M2M关系

Question

I have a model like this

class Authority(models.Model):
    name=models.CharField(max_length=100)
    country=models.ForeignKey(Country)
    category=models.ForeignKey(Category)
    competitors=models.ManyToManyField("self",related_name="competitors")

I want authorities having the same country and category and itself to be automatically give an M2M relationship,so i did this

def save(self,*args,**kwargs):
    z=Authority.objects.filter(country=self.country).filter(category=self.category)
    this_authority=Authority.objects.get(id=self.id)
    for a in z:
        this_authority.competitors.add(a)
    super(Authority,self).save(*args,**kwargs)

It wasn't working and not bringing any error,I also tries this below

def save(self,*args,**kwargs):
        z=Authority.objects.filter(country=self.country).filter(category=self.category)
        this_authority=Authority.objects.get(id=self.id)
        self.competitors=z
        super(Authority,self).save(*args,**kwargs)

What might be wrong with my code?Thanks in advance.

Answer 1

The reason this isn't working the way you expect is because of how Django handles creating m2m relationships in the database. Long story very short, when you save something like a new Authority to the database, Django first writes the new object then goes back in and writes the m2m relationships for that new object. As a result, it's tough to do anything useful to m2m relationships in a custom save method.

A post-save signal may do the trick here. kwargs['created'] = True if we're creating a new object and kwargs['instance'] is the instance whose save fired off the signal receiver.

@receiver(post_save, sender = Authority)
def update_m2m_relationships(sender, **kwargs):
    if kwargs['created']: #only fire when creating new objects
        competitors_to_add = Authority.objects.filter(
                                country = kwargs['instance'].country, 
                                category = kwargs['instance'].category
                                )
        for c in competitors_to_add:
            c.competitors.add(kwargs['instance'])
            c.save() #not creating a new object; this receiver does not fire here
            kwargs['instance'].competitors.add(c)
        #all competitors have been added to the instance's m2m field
        kwargs['instance'].save()

It's important to only fire this when creating new objects. If you don't include that restriction, then the receiver will trigger itself as you update other objects in your for loop.

I haven't tested this out but I think it'll work. Let me know if it doesn't and I'll do my best to help.