Custom Fieldtype with Checkboxes - Remove value and set to null when checkboxes are left blank I created a custom field type to show user groups in a checkbox layout, e.g. (http://cl.ly/image/0A1a140m031h).

I am running into one small issue. If I add content with a checkbox(s) option selected and save the entry. Then I try to edit the entry and uncheck all the options, the entry will still have the original checkbox options selected and won't return to the null state as expected. I was trying to use prepValueFromPost but that doesn't seem to run unless the value is set. How do I fix this?
This isn't really an answer. But I think I do know what might be happening. The default browser behavior is that checkbox values only get submitted when it has the attribute 'checked' (why? I have no idea). In php you're essentially testing whether the post value exists or not. If it exists, true; if not, false. I assume for the built-in checkbox fieldtype that craft does this for you in the background (by somehow comparing with the original options). You'll have to figure out a way to perform this check yourself. Maybe take a look at the source files and see how craft is handling this.