Django: use related_name to create a subclass-CodePudding

How can I be able to use related_name to create a child model when the parent model is being created.

from django.db import models

from django.db.models.signals import pre_save, post_save
from django.dispatch import receiver


class Parent(models.Model):
    name = models.TextField()
    child = models.ForeignKey(Child, on_delete=models.SET_NULL, related_name="parent_child", null=True, blank=True)

    def save(self, *args, **kwargs):
        super().save()

@receiver(post_save, sender=Parent)
def post_save_receiver(sender, instance, *args, **kwargs)
    if not instance.child:
        instance.child.parent_child.create(name="I am a child")


class Child(models.Model):
    name = models.CharField()

Currently, on save, I get a NoneType error:

'NoneType' object has no attribute 'parent_child'

These are the local variables in my stack trace

args: ()

instance: <Parent: asdf - asd-47013855>

kwargs:
{
 'created': True,
 'raw': False,
 'signal': <django.db.models.signals.ModelSignal object at 0x7f20b92e2ce0>,
 'update_fields': None,
 'using': 'default'
}

sender: <class 'parent.models.Parent'>

CodePudding user response：

def post_save_receiver(sender, instance, *args, **kwargs)
    if not instance.child:
        child = Child(name='I am a child').save()
        instance.child = child
        instance.save()

CodePudding user response：

You're trying to perform action on NoneType object eg. instance.child because when you create your Parent instance you've to assing child to your Parent.child attribute so by default it will None so to assign child to your Parent class you can do like this & you don't require signals you can do this inside your save method.

class Parent(models.Model):
    name = models.TextField()
    child = models.ForeignKey(Child, on_delete=models.SET_NULL, related_name="parent_child", null=True, blank=True)

    def save(self, *args, **kwargs):
        if not self.child:
           self.child = Child.objects.create(name="I am a child")
        super(Parent, self).save(*args, **kwargs)

also you're trying to access related object in wrong way if you're inside Parent you can access child directly using Parent.child & if you want to access Parent from Child then you can do like this Child.parent_child read more about backward relation