django auto-create intermediate model instances for new instance created in foreignkey field model

Question

There are two parts of this question for following situation:

I will use example to describe the question:

These are my django models:

class Zone(models.Model):
    zone_name = models.CharField(max_length = 10)
    zone_number = models.CharField(max_length = 10)

    class Meta:
        ordering = ('zone_name',)      

    def __unicode__(self):
        return self.zone_name  


class Stage(models.Model):
    stage_number = models.CharField(max_length = 10)
    stage_name = models.CharField(max_length = 10)
    zones = models.ManyToManyField(Zone, through='ZoneStage')

    class Meta:
        ordering = ('stage_number',)      

    def __unicode__(self):
        return self.stage_number  

    @property
    def value(self):
        return ZoneSubStage.objects.filter(substage__stage=self).aggregate(Sum('value')).get('value__sum', 0)  


class ZoneStage(models.Model):
    zone = models.ForeignKey(Zone)
    stage = models.ForeignKey(Stage)
    value = models.PositiveSmallIntegerField(default=0)

    class Meta:
        ordering = ('zone',)          

    def __unicode__(self):
        return '%s %s' % (self.zone, self.stage)

class SubStage(models.Model):
    sub_name = models.CharField(max_length=10)
    stage = models.ForeignKey(Stage)
    zones = models.ManyToManyField(Zone, through='ZoneSubStage')

    def __unicode__(self):
        return self.sub_name


class ZoneSubStage(models.Model):
    zone = models.ForeignKey(Zone)
    substage = models.ForeignKey(SubStage)
    value = models.PositiveSmallIntegerField(default=0)

    def __str__(self):
        return '%s %s' % (self.zone, self.substage)  

QUESTION 1: How to create automatically new ZoneStage instances for all related Stages if I create manually new Zone instance ?

e.g. I have following Stage instances: S1, S2, S3, S4 and I create new Zone instance "A". So, I want to create automatically new ZoneStage instances AS1, AS2, AS3, AS4 IF such instance has not existed already ?

QUESTION 2: This is the extension to Question 1. If I create new Zone I want to create automatically ZoneStage instances as described in Question 1 plus I want to create automatically ZoneSubStage instances for all SubStages in all Stages if such ZoneSubStage instance has not existed already.

I do not know how to start with it. I though I should read maybe about post_save() first ?

Answer 1

Sub answer : Firstly you don't want to add zone = models.ForeignKey(Zone) in class ZoneStage because you have this information with stage = models.ForeignKey(Stage);

Question 1: you can intercept signal (post_save) after create stage like this.

@receiver(post_save, sender=Stage)
def gen_coupon(sender, instance, raw=True, **kwargs):
    new_zone_stage = ZoneStage(stage=instance)
    new_zone_stage.save()
    return kwargs

Question 2: Same logic but you change sender to current created model object

Answer 2

I have read about my problem and gathered some knowledge from django manuals. Rakwen advice pointed me towards right direction. This is what I have and this is working at the moment. Although, I do not check if such instance already exists. Furthermore, I have read in two scoop of django it is not good to use signals unless it is really really necessary. I should use model managers instead.

I have created file signals.py and stored it in my app folder next to models.py. It contains following code:

from django.dispatch import Signal


def create_new_zonestage_if_new_zone(sender, **kwargs):
    if kwargs.get('created',False):
        from models import Stage, ZoneStage
        stages = Stage.objects.all()
        for mystage in stages: 
            ZoneStage.objects.get_or_create(stage=mystage,zone=kwargs.get('instance'))
    return kwargs

def create_new_zonesubstage_if_new_zone(sender, **kwargs):
    if kwargs.get('created',False):
        from models import SubStage, ZoneSubStage
        substages = SubStage.objects.all()
        for mysubstage in substages: 
            ZoneSubStage.objects.get_or_create(substage=mysubstage,zone=kwargs.get('instance'))
    return kwargs

I have added following into models.py:

above model classes:

from django.db.models.signals import post_save
from . import signals

below model classes:

post_save.connect(signals.create_new_zonestage_if_new_zone, sender=Zone)
post_save.connect(signals.create_new_zonesubstage_if_new_zone, sender=Zone)

it works.