Ok, I made hopefully a less egregious hack. I agree that the right solution is for the client to not use null to indicate that it wants to delete fields, but the module_editor.coffee would need significant fleshing out to fix this; so, instead of adding a 'to_delete' field in the post, I added a 'nullout' field and fill it with ['markdown'] when markdown should be nulled out (those models were fleshed out).