Django admin: how do I add an unrelated model field to a model change/add page?

Question

I have the following models:

class Foo(models.Model):
    field1 = models.IntegerField()
    ...

class Bar(models.Model):
    field1 = models.IntegerField()
    ...

class Foo_bar(models.Model):
    foo = models.ForeignKey(Foo)
    bar = models.ForeignKey(Bar)
    ...

In the admin, I want it so that in the Foo change/add page, you can specify a Bar object, and on save I want to create a Foo_bar object to represent the relationship. How can I do this through customizing the Admin site/ModelAdmins? Note that inlining isn't quite what I need because there is no explicit foreign key relationship between foo and bar. And second, I don't actually want to edit bar objects, I just want to choose from amongst the ones that are in the system.

Thanks.

Answer 1

Are you sure you don't just want a ManyToManyField? In the admin, this would manifest as a multiple-selection list of Bar objects. Look at the group selection part of the admin for User.

If you need additional data attached to the relationship, you could use a through parameter:

class Foo(models.Model):
    field1 = models.IntegerField()
    bars = models.ManyToManyField("Bar", related_name="foos", through="Foo_bar")

You'll need to add Foo_bar to the admin in order to edit these additional parameters in the admin.

Answer 2

I can think of two possibilities:

1. You could create a custom form for your Foo model and add a field containing the Bar.objects.all() queryset to it. then override the ModelAdmin's default save_form() method to create a new Bar instance upon saving the object.

2. You could create a custom Field class and add it to you Foo model, and class this functionality via a ´post_save` signal...