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?