Link to child to parent in Django model - django

Question

I have a Place Model:
class Place(models.Model):
name = models.CharField(max_length=200, null=True, blank=True)
country = models.CharField(max_length=100, null=True, blank=True)
postal_code = models.CharField(max_length=100, null=True, blank=True)
region = models.CharField(max_length=1000, choices=REGION, null=True, blank=True)
longitude = models.CharField(max_length=20, null=True, blank=True)
latitude = models.CharField(max_length=20, null=True, blank=True)
date_created = models.DateTimeField(_('date created'), default=timezone.now)
date_modified = models.DateTimeField(_('date_modified'), auto_now=True)
Originally I create place to save information about a city, like San Francisco or New York.
Now I want to create a new model called Bar:
class Bar(Place):
location = models.OneToOneField(Place, verbose_name=_('location'), related_name='bar', blank=True, on_delete=models.PROTECT, parent_link=True)
date_inception = models.DateField(null=True, blank=True)
date_closed = models.DateField(null=True, blank=True)
I would like to link the bar to a particular city in the Place model. So I'm linking one row of the an inherited model with its parent. Can I avoid creating a city model and the Bar should Foreignkey into? Is this doable? If so, how do I do it?

Answer 1

When you set parent_link=True in your OneToOneField, the joined models are treated as the same model when working with the ORM, even though they are two separate database tables.
There is no need to link Bar to a particular Place; you can use the Bar model to create both the Bar instance and the Place instance at the same time:
bar = Bar.objects.create(name='Blackbird', city='San Fransisco',
country='US', date_inception='10-17-2019', ...
)
Based on your comments, I'm guessing you don't want this at all. Instead, you probably want a ForeignKey relationship between Place and the Bar model, since a place (eg San Fransisco) can have many bars, but a bar can only have one place.
Update
From my understanding of your comments, this is what I would suggest:
class PlaceMixin(models.Model):
name = models.CharField(max_length=200, null=True, blank=True)
longitude = models.CharField(max_length=20, null=True, blank=True)
latitude = models.CharField(max_length=20, null=True, blank=True)
class Meta:
abstract = True
def save(self, *args, **kwargs):
latitude = self.latitude
longitude = self.longitude
do_stuff(latitude, longitude)
super().save()
class Location(PlaceMixin):
country = models.CharField(max_length=100, null=True, blank=True)
postal_code = models.CharField(max_length=100, null=True, blank=True)
region = models.CharField(max_length=1000, choices=REGION, null=True, blank=True)
class Bar(PlaceMixin):
location = models.ForeignKey(Location, related_name='bars', on_delete=models.CASCADE)
class Store(PlaceMixin):
location = models.ForeignKey(Location, related_name='stores', on_delete=models.CASCADE)
Now you can access both the lat/long of the Place, and of the Location:
>>>location = Location.objects.create(name='San Fransisco', latitude=1, longitude=1)
>>>bar = Bar.objects.create(name='Blackbird', latitude=2, longitude=2, location=location)
>>>
>>>bar.latitude
2
>>>bar.location.latitude
1
To query all bars in San Fransisco:
location = Location.objects.select_related('Bar').get(name='San Fransisco')
bars = location.bars.all()