How to model a m2m relation where the related table can be a city, a region (state) or a country

Question

In Django I have theses models:

class Artist(models.Model):
    name = models.CharField(max_length=128)
    born_place = models.ForeignKey(???)
    dead_place = models.ForeignKey(???)
    live_places = models.ManyToManyField(???)
    work_places = models.ManyToManyField(???)

class Country(models.Model):
    iso = models.CharField(max_length=2, primary_key=True)
    name = models.CharField(max_length=50)

class Region(models.Model):
    iso = models.CharField(max_length=2, blank=True)
    name = models.CharField(max_length=150)
    country = models.ForeignKey('Country')

class City(models.Model):
    name = models.CharField(max_length=150)
    region = models.ForeignKey('Region')

All the places (born_place, dead_place, live_places, work_places) can be a City or a Region or a Country. And a City should necessarily have a Region, and a Region should necessarily have a Country.

How can I achieve that?

Answer 1

All the places (born_place, dead_place, live_places, work_places) can be a City or a Region or a Country.

What you need in this case is a generic foreign key. In Django this can be achieved through the handy contenttypes framework.

From the documentation:

There are three parts to setting up a GenericForeignKey:

1. Give your model a ForeignKey to ContentType.

2. Give your model a field that can store a primary-key value from the models you'll be relating to. (For most models, this means an IntegerField or PositiveIntegerField.) This field must be of the same type as the primary key of the models that will be involved in the generic relation. For example, if you use IntegerField, you won't be able to form a generic relation with a model that uses a CharField as a primary key.

3. Give your model a GenericForeignKey, and pass it the names of the two fields described above. If these fields are named "content_type" and "object_id", you can omit this -- those are the default field names GenericForeignKey will look for.

Answer 2

Take a look at Django's Generic Relations here and here, which will let you key to various types of object, rather than only a specific one.

Answer 3

well.. yea maybe you could use generic relations. But i think that you could solve you some problems by thinking outside the box.

i'd say that you could create a Territory model instead of Country,Region and City; and then use a recursive relationship

class Territory(models.Model):
   iso = models.CharField(max_length=2, blank=True)
   name = models.CharField(max_length=150)
   parent = models.ForeignKey('self')

this will create a hierarchy structure for your territory, so now you could use as many divisions as you like (for exemple, Continents, Planets,Community) and you wont have to change your models. As for the Artist, you could do something like:

class Artist(models.Model):
    name = models.CharField(max_length=128)
    born_place = models.ForeignKey(Territory)
    dead_place = models.ForeignKey(Territory)
    live_places = models.ManyToManyField(Territory)
    work_places = models.ManyToManyField(Territory)

so now... the born_place can be a City, a Region, a Planet... anything you want! i guess that was your question. I'm no expert in django, this is just a generic way to solve this kind of problems in O.O.P.

Answer 4

You can also use model inheritance

class Territory(models.Model):
    name = models.CharField(max_length=150)

class Country(Territory):
    iso = models.CharField(max_length=2, primary_key=True)

class Region(Territory):
    iso = models.CharField(max_length=2, blank=True)
    country = models.ForeignKey('Country')

class City(Territory):
    region = models.ForeignKey('Region')

class Artist(models.Model):
    name = models.CharField(max_length=128)
    born_place = models.ForeignKey(Territory)
    dead_place = models.ForeignKey(Territory)
    live_places = models.ManyToManyField(Territory)
    work_places = models.ManyToManyField(Territory)

Best regards!