rels.py

# models.py

class Subscription(models.Model):
    pass

class User(models.Model):
    subscription = models.ForeignKey(Subscription, null=True,
                                     blank=True, related_name='users',
                                     on_delete=models.SET_NULL)


# admin.py

class SubscriptionAdminForm(forms.ModelForm):
    class Meta:
        model = Subscription
        fields = ()

    # users = make_ajax_field(Subscription, 'users', 'user')
    users = AutoCompleteSelectMultipleField('user', required=False)

    def __init__(self, *args, **kwargs):
        super(SubscriptionAdminForm, self).__init__(*args, **kwargs)
        if self.instance:
            self.fields['users'].initial = self.instance.users.all()


class SubscriptionAdmin(admin.ModelAdmin):
    form = SubscriptionAdminForm

    def save_model(self, request, obj, form, change):
        # TODO: do changes in a transaction
        super(SubscriptionAdmin, self).save_model(request, obj, form, change)
        form.fields['users'].initial.update(subscription=None)
        User.objects.filter(
            id__in=form.cleaned_data['users']).update(subscription=obj)


admin.site.register(Subscription, SubscriptionAdmin)