How to access both directions of ManyToManyField in Django Admin?

Question

The Django admin filter_horizontal setting gives a nice widget for editing a many-to-many relation. But it's a special setting that wants a list of fields, so it's only available on the (admin for the) model which defines the ManyToManyField; how can I get the same widget on the (admin for the) other model, reading the relationship backwards?

My models look like this (feel free to ignore the User/UserProfile complication; it's the real use case though):

class Site(models.Model):
    pass
class UserProfile(models.Model):
    user = models.OneToOneField(to=User,unique=True)
    sites = models.ManyToManyField(Site,blank=True)

I can get a nice widget on the admin form for UserProfile with

filter_horizontal = ('sites',)

but can't see how to get the equivalent on the Site admin.

I can also get part-way by adding an inline to SiteAdmin, defined as:

class SiteAccessInline(admin_module.TabularInline):
    model = UserProfile.sites.through

It's roundabout and unhandy though; the widget is not at all intuitive for simply managing the many-to-many relationship.

Finally, there's a trick described here which involves defining another ManyToManyField on Site and making sure it points to the same database table (and jumping through some hoops because Django isn't really designed to have different fields on different models describing the same data). I'm hoping someone can show me something cleaner.

Answer 1

Here's a (more or less) tidy solution, thanks to http://blog.abiss.gr/mgogoulos/entry/many_to_many_relationships_and and with a fix for a Django bug taken from http://code.djangoproject.com/ticket/5247

from django.contrib import admin as admin_module

class SiteForm(ModelForm):
    user_profiles = forms.ModelMultipleChoiceField(
        label='Users granted access',
        queryset=UserProfile.objects.all(),
        required=False,
        help_text='Admin users (who can access everything) not listed separately',
        widget=admin_module.widgets.FilteredSelectMultiple('user profiles', False))

class SiteAdmin(admin_module.ModelAdmin):
    fields = ('user_profiles',)

    def save_model(self, request, obj, form, change):
        # save without m2m field (can't save them until obj has id)
        super(SiteAdmin, self).save_model(request, obj, form, change) 
        # if that worked, deal with m2m field
        obj.user_profiles.clear()
        for user_profile in form.cleaned_data['user_profiles']:
             obj.user_profiles.add(user_profile)

    def get_form(self, request, obj=None, **kwargs):
        if obj:
            self.form.base_fields['user_profiles'].initial = [ o.pk for o in obj.userprofile_set.all() ]
        else:
            self.form.base_fields['user_profiles'].initial = []
        return super(SiteAdmin, self).get_form(request, obj, **kwargs)

This uses the same widget as the filter_horizontal setting, but hard-coded into the form.