Update a field based on the value of another

I'm loving DataTables and have only encountered one complication. I have a scenario where a field is updated by the value of a group of three radio boxes. This works fine. However on edit/update, I wish to check this value, and based on it, update another field within the Database.

In this instance, I wish to update a field "reservation_id" to 0 if "status" is updated to "Available"

My current code is:

Editor::inst( $db, 'apartments' )
	->fields(
		Field::inst( 'number' ),
		Field::inst( 'level' ),
		Field::inst( 'price' ),
		Field::inst( 'status' )
            ->setFormatter( function ($val, $data, $field) {
                if ($val == "Available") return 0;
				else if ($val == "Reserved") return 1;
				else return 2;
            } )
            ->getFormatter( function ($val, $data, $field) {
                if ($val == 0) return "Available";
				else if ($val == 1) return "Reserved";
				else return "Sold";
            } ),
		Field::inst( 'test' )
			->set("blah")
		)
	->join(
		Join::inst( 'reservations', 'object' )
			->join( 'id', 'apartment_id' )
			->field(
				Field::inst( 'first_name' ),
				Field::inst( 'last_name' ),
				Field::inst( 'email' ),
				Field::inst( 'phone' ),
				Field::inst( 'initial_deposit' ),
				Field::inst( 'agent_company' ),
				Field::inst( 'agent_name' ),
				Field::inst( 'agent_email' ),
				Field::inst( 'date' )
					->getFormatter( 'Format::date_sql_to_format', 'D, d M y' )
			)
		)
	
	->process( $_POST ) // The "process" method will handle data get, create, edit and delete requests from the client
	->json();

Thanks!