UPDATE Aug, 2011: This patch added a new hook to ModelAdmin called save_related(). You don’t need the hack described bellow anymore :).

Admin is an awesome Django builtin app to create nice CRUDs for your models, and offers a lot of customizations hooks. You can personalize the templates, perform custom filters, modify newly created objects and if that’s not enough, you can always create your own view to do something it doesn’t by default.

The admin docs are great, I’m not going to explain how it works. The intention here is to show a way to modify an object, after it’s saved, and other objects related to it using inlines are saved too.

Once you’ve configured your admin interface with inlines, you end up with something similar to:

from django.contrib import admin
from fooapp.models import Foo, Related

class RelatedInline(admin.TabularInline):
    model = Related

class FooAdmin(admin.ModelAdmin):
    inlines = [RelatedInline]

admin.site.register(Foo, FooAdmin)

this mean whens you’re adding a Foo, since Related has a foreign key to it, django will display a few forms to add Relateds in the same page.

Imagine you need to do something with the new foo instance that needs to now how many related objects it has. Admin already has methods you can override to do something after Foo is saved, like ModelAdmin.save_model(). See how it works:

class FooAdmin(admin.ModelAdmin):
    inlines = [RelatedInline]

    def save_model(self, request, obj, form, change):
        # do something with obj.related_set.all()
        # OPS! it's empty!

admin.site.register(Foo, FooAdmin)

the problem here is that save_model() is called before the inlines are saved.

Let’s find out how it works. Open django source code, specifically in django.contrib.admin.options.py go to add_view(), it’s the view called when you are creating an object. As you can see, when the request method is “POST” it goes through all validation, for the main form and all related formsets (your inlines). And if all of then are valid it calls save_model(), then save_formset() for each formset. The interesting piece is shown below:

class ModelAdmin(BaseModelAdmin):

    # ...

    def add_view(self, request, form_url='', extra_context=None):

        # ... all validation here ...

        if all_valid(formsets) and form_validated:
            self.save_model(request, new_object, form, change=False)
            for formset in formsets:
                self.save_formset(request, form, formset, change=False)

            self.log_addition(request, new_object)
            return self.response_add(request, new_object)

Notice here that the last method it calls is response_add(), and it passes the created object, that’s all we need! If you see the change_view() method (witch it the view called when you’re editing an object) it calls a similar method: response_change().

Now we can solve our problem doing something like this:

class FooAdmin(admin.ModelAdmin):
    inlines = [RelatedInline]

    def response_add(self, request, new_object):
        obj = self.after_saving_model_and_related_inlines(new_object)
        return super(FooAdmin, self).response_add(request, obj)

    def response_change(self, request, obj):
        obj = self.after_saving_model_and_related_inlines(obj)
        return super(FooAdmin, self).response_change(request, obj)

    def after_saving_model_and_related_inlines(self, obj):
        print obj.related_set.all()
        # now we have what we need here... :)
        return obj

There are two things to keep in mind when using these methods:

  • They are not documented. So probably it’s not part of the API admin expects you to override. Thankfully django publishes excellent release notes including backwards incompatible changes. And I hope you have good test coverage to know if it will break when you update django :)
  • Their intention is to handle the response of the view, as it docstring says: “Determines the HttpResponse for the add_view stage”. So if you do something there not related to it, it works, wont make much sense if you look at the overall architecture.

Maybe django will add a more specific hook to solve this issue later, but I hope it helps you for now.