Custom field (multiselect) data deleted on bulk edit

[m2martin]

NetBox version

v3.0.3

Python version

3.9

Steps to Reproduce

1. Create a custom field of type "multiple selection", add some values and apply it to any content type (e.g. interfaces)
2. Add some custom values to one or more objects the custom field is applied to
3. Bulk edit one or more objects where custom values have been applied to
4. In the bulk edit form click "Apply" without editing anything

Expected Behavior

As the custom fields have not been edited, the values should be preserved.

Observed Behavior

The custom field data is deleted.

[m2martin]

Form data for custom fields is checked in:

netbox/netbox/netbox/views/generic.py

Lines 830 to 835 in 6bc7210

	# Update custom fields
	for name in custom_fields:
	if name in form.nullable_fields and name in nullified_fields:
	obj.custom_field_data[name] = None
	elif form.cleaned_data.get(name) not in (None, ''):
	obj.custom_field_data[name] = form.cleaned_data[name]

Custom field data is only changed if form field is in nullified_fields or if its data is not empty (None or '').
Actually, the unmodified (empty) form field data is an empty list [], so elif is True and sets custom field data to [].

Tho following line 834 would correct this issue.

elif form.cleaned_data.get(name) not in (None, '', []):

[m2martin]

If my proposal is acceptable, feel free to assign it to me.

[jeremystretch]

Thanks @m2martin. That's definitely the right track, though I think the original implementation can be improved a bit further by evaluating form.changed_data.