CODEWITHSUNDEEP
pythondjangodjango-models
Milano
Milano

Reputation: 18745

Django OneToOne field to self

How to define OneToOne relationship to the same Model?

I have a model called Order which can be paired with another one Order. Now I'm trying to figure out how to handle models for this relationship.

My ideas:

class Order(models.Model):
    paired_order = models.OneToOneField(self)

OR:

class Pairing(models.Model):
    order1 = models.ForeignKey(Order, related_name='pairing')
    order2 = models.ForeignKey(Order, related_name='pairing')

What do you think? Which is more efficient?

I want to have simple calling of paired ones. So I would do something like:

order.paired_order

OR:

order.pairing.paired

I want this relation symmetrical so for each pair of orders I call the same thing and get paired order.

Pairing model would be a good solution because I can add additional information to this relationship, but there is a problem that I would have to detect which order is it, so I couldn't call order.pairing.order1 because I don't know whether I'm not calling the same order.

EDIT: 

>>> from _app import models
>>> order1 = models.Order(flight_number="xxx")
>>> order2 = models.Order(flight_number="yyy", paired_order=order1)
>>> order1.paired_order.flight_number

RETURNS None object has not ....

The problem is that when I set order1 is a paired order for order2, I want the same thing in opposite direction. So order1.paired_order = order2 do this as well order2.paired_order = order1.

Upvotes: 6

Views: 5337

Answers (3)

Renoc
Renoc

Reputation: 439

Having this problem myself, the term 'symmetrical' was key to finding the answer: https://code.djangoproject.com/ticket/7689

class Order(models.Model):
    paired_order = models.OneToOneField(self)

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

Upvotes: 3

François Constant
François Constant

Reputation: 5496

Pairing model would be a good solution because I can add additional information to this relationship.

In that case, you could model that group of "orders" (you've called it Pairing) and add a shortcut to retrieve the paired order.

class OrderPair(models.Model):
    pass        
    # additional information goes here


class Order(models.Model):
    pair = models.ForeignKey(to="OrderPair", related_name="orders")
    # you'll have to add custom validation 
    # to make sure that only 2 orders can belong to the same "OrderPair"

    @property
    def paired_order(self):
         return self.pair.orders.exclude(id=self.id).first()

Once you've got this working, you might also want to cache the paired order to avoid too many queries. In that case, you don't want a related name so you can use + (the less explicit thing in Django ever).

class Order(models.Model):
    ...
    cached_paired_order = models.ForeignKey(to='self', related_name='+')

@property
def paired_order(self):
     if self.cached_paired_order:
          ...
     else:
          ...

Upvotes: 3

zsepi
zsepi

Reputation: 1662

The ForeignKey accepts as an argument not just a class, but also a string name of the form ForeignKey('ModelNameInSameModelsPyFile') or ForeignKey('app_name.ModelName).

In your case, it could be like

class Order(models.Model):

     paired = models.ForeignKey('Order', null=True)

You can read more at https://docs.djangoproject.com/en/1.8/ref/models/fields/#foreignkey

Upvotes: 1

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