don't set m2m on objects that can have no groups

src/c3nav/editor/forms.py

@@ -4,6 +4,7 @@ from collections import OrderedDict
 from functools import reduce
 
 from django.conf import settings
+from django.core.exceptions import FieldDoesNotExist
 from django.forms import BooleanField, CharField, ModelForm, MultipleChoiceField, ValidationError
 from django.forms.widgets import HiddenInput
 from django.utils.translation import ugettext_lazy as _
@@ -116,6 +117,12 @@ class MapitemFormMixin(ModelForm):
 
     def _save_m2m(self):
         super()._save_m2m()
+        try:
+            field = self._meta.model._meta.get_field('groups')
+        except FieldDoesNotExist:
+            pass
+        else:
+            if field.many_to_many:
                 groups = reduce(operator.or_, (set(value) for name, value in self.cleaned_data.items()
                                                if name.startswith('groups_')), set())
                 groups = tuple((int(val) if val.isdigit() else val) for val in groups)