Python/Django models

Question

I am fairly new to Python/Django. What I would like to do is to store descriptions of cars separately, but simultaneously I would like to label (in django admin) car description like this:

class CarDescription(models.Model):
 length = models.IntegerField(max_length=10)
 def __unicode__(self):
 return "description of the car no %d" % (Car.id)
class Car(models.Model):
 description = models.OneToOneField(CarDescription)

I know that Car.id is wrong there (circular reference). Is there any way to solve it?

• Andrey: Why would you want to store it separately, if there is 1-to-1 relationship? It will make your life harder with no benefits.
• JosephConrad: Well, I would like to split it, because it can have potentially many fields. I would prefer to have two entities with smaller amount of attributes rather than one with many attributes.
• Andrey: what is your reasoning behind it? You have to have real good reasons for such insane normalization.
• Andrey: ok, if you want it THAT much, just add string field to CarDescription and that's it. But seriously, I doubt it is reasonable.
• JosephConrad: Ok, so you suggest to split class only in case of some kind of inheritance? And generally not to introduce one-to-one relation?

Answer 1

You should structure your models like this:

class Car(models.Model):
 # everything that has to do _only_ with a car
class CarDescription(models.Model):
 car = models.ForeignKey(Car) # each description can belong to only one car
 length = models.IntegerField(max_length=10)
 # other fields that have only to do with a description
 def __unicode__(self):
 return unicode("description of the car no %d" % (self.car.pk))

Django has a very nice API for looking up related objects which you should go through (once you have finished the tutorial).

Answer 2

All you need is:

class Car(models.Model):
 description = models.CharField(max_length=20)

Done. More fields are fine. You're overcomplicating things otherwise.

You need to study what's called "relational modeling".

What you're doing is "premature optimization" and probably "pessimization".

Answer 3

You can check django's document, https://docs.djangoproject.com/en/dev/topics/db/queries/#one-to-one-relationships.

As addressed in the doc, you can access one to one fields directly,

class CarDescription(models.Model):
 length = models.IntegerField(max_length=10)
 def __unicode__(self):
 return "description of the car no %d" % (self.car.id)

It works only both car and car description instances exists, or else exception will be thrown.