Well, I still didn't feel completely comfortable with Tim Edgar's solution, so I kept looking. I guess I found what I was looking for. The 'Form' class, has two undocumented methods that are of use in this case: 'has_changed()' and '_get_changed_data'.
During ModelFormSet validation, every form checks 'has_changed()'. If the form did not changed, validation is skipped and a correct form is assumed. Likewise, during ModelFormSet saving, the save_new_objects checks every form to see if it has changed. If it didn't change, the form isn't saved.
So my solution was to override the has_changed() method to return False if only the 'group' attribute has changed, and all other fields are empty. This is my implementation:
class UsesIngredientForm(forms.ModelForm):
class Meta:
model = UsesIngredient
def has_changed(self, *args, **kwargs):
self._get_changed_data(*args, **kwargs)
# If group is in changed_data, but no other fields are filled in, remove group so
# the form will not be validated or saved
if 'group' in self._changed_data and len(self._changed_data) == 1:
contains_data = False
for name in ['ingredient', 'amount', 'unit']:
field = self.fields[name]
prefixed_name = self.add_prefix(name)
data_value = field.widget.value_from_datadict(self.data, self.files, prefixed_name)
if data_value:
contains_data = True
break
if not contains_data:
self._changed_data.remove('group')
return bool(self._changed_data)
Hope this helps anybody in the future!
EDIT: I edited this answer to reflect Tim Edgars comment. I realize that this implementation still uses 'private' methods, but I haven't found a cleaner implementation using just the publicly documented methods. But then maybe that is just my own incompetence :).