Custom FilterSpec to limit groups to that of the user in list_filter

Sub-classed Django's RelatedFilterSpec and filtered the available groups
based on the current user's groups. This now overrides the standard
FilterSpec for the groups field.

password/admin.py

@@ -1,10 +1,54 @@
 from django.contrib import admin
+from django.contrib.admin.filterspecs import FilterSpec, RelatedFilterSpec
 from django.contrib.auth.models import Group
 from django.db.models import Q
+from django.utils.encoding import smart_unicode
+from django.utils.translation import ugettext_lazy as _
 
 from forms import PasswordForm
 from models import Password
 
+class UserGroupsFilterSpec(RelatedFilterSpec):
+    """
+    Custom filter spec taken from:
+    http://stackoverflow.com/questions/2251851/django-admin-list-filter-attribute-from-userprofile
+    """
+    def __init__(self, f, request, params, model, model_admin, field_path=None):
+        super(UserGroupsFilterSpec, self).__init__(f, request, params, model, model_admin, field_path=field_path)
+
+        # The lookup string that will be added to the queryset by this filter
+        self.lookup_kwarg = 'group__id__exact'
+        # get the current filter value from GET (we will use it to know which filter item is selected)
+        self.lookup_val = request.GET.get(self.lookup_kwarg)
+
+        # A list of the user's groups, ordered alphabetically, containing only the id and name.
+        self.lookup_choices = Group.objects.filter(user=request.user).order_by('name').values_list('id', 'name')
+
+    def choices(self, cl):
+        """
+        Generator that returns all the possible items in the filter including 'All' and 'Private' items.
+        """
+        yield { 'selected': self.lookup_val is None,
+                'query_string': cl.get_query_string({}, [self.lookup_kwarg]),
+                'display': _('All') }
+        for pk_val, val in self.lookup_choices:
+            yield { 'selected' : self.lookup_val == smart_unicode(pk_val),
+                    'query_string': cl.get_query_string({self.lookup_kwarg: pk_val}),
+                    'display': val }
+
+    def title(self):
+        return _('Group')
+
+# Here, we insert the new FilterSpec at the first position, to be sure it gets picked up before any other.
+FilterSpec.filter_specs.insert(0,
+    (lambda f: getattr(f, 'usergroup_filter', False), UserGroupsFilterSpec)
+)
+
+# Add the usergroup filter by setting the usergroup_filter attribute on an existing field.
+# This will activate the user groups filter if we add it to the admin's `list_filter`,
+# however we won't be able to use it in it's own filter anymore.
+Password._meta.get_field('group').usergroup_filter = True
+
 class PasswordAdmin(admin.ModelAdmin):
     form = PasswordForm
     list_display = ('__unicode__', 'domain', 'is_public')