python - Django model one foreign key to many tables

Question

So I have a question I was thinking of creating a single table that has a foreign key to several other tables, and using another field "type" to say what table the key should belong to.

class Status(Models.model):
    request = models.ForeignKey("Request1", "Request2", "Request3")
    request_type = models.IntegerField()
    ...Some status related data

class Request1(Models.model):
    ...Some data

class Request2(Models.model):
    ...Some other data

Class Request3(Models.model):
   ...different data

My question is, is it possible to define a foreign key like this? another solution I thought of was to define my model like this

class Status(Models.model):
    request1 = models.ForeignKey("Request1")
    request2 = models.ForeignKey("Request2")
    request3 = models.ForeignKey("Request3")
    ...Some status related data

class Request1(Models.model):
    ...Some data

class Request2(Models.model):
    ...Some other data

Class Request3(Models.model):
   ...different data

But if I do it this way is it possible to define a constraint via django that says only 1 foreign key is allowed to have data and the other two must be null? or will I have to strictly set this constraint up on the db side.(I'm using postgres) I would like to be able to tell django to do it when it creates the db so I don't have to remember every time someone recreates the db.

Any input or advice would be greatly appreciated. I am not married to either of these ideas, so if there is another clever way to achieve the same effect i am up to hear it. Thank you for your time.

Edit: I am using django 1.7.10

Answer 1

You should use the contentypes framework in Django.

There's an example for a generic relation here :https://docs.djangoproject.com/en/1.8/ref/contrib/contenttypes/#generic-relations For your requirement it could look something like this:

from django.db import models
from django.contrib.contenttypes.fields import GenericForeignKey
from django.contrib.contenttypes.models import ContentType

class Status(models.Model):
    request_type = models.ForeignKey(ContentType)
    request_id = models.PositiveIntegerField()
    request = GenericForeignKey('request_type', 'request_id')

You can then do something like following:

status1 = Status(request=Request1("foo"))
status1.save()
status2 = Status(request=Request2("bar"))
status2.save()

status1.request // <Request1 "foo">
status2.request // <Request2 "bar">