overwrite new indexes with previous json value

lib/dbwrap.js

@@ -193,8 +193,15 @@ function wrap(db, dir, dbsMap) {
         delete row[idname];
 
         Object.keys(row).forEach(function (fieldname) {
-          // TODO warn if overriding proper field? (shouldn't be possible)
+          // Ideally it shouldn't be possible to overriding a former proper column,
+          // but when a new indexable field is added, the old value is still in json
+          // TODO one-time upgrade of all rows when a new column is added
+          if (null === row[fieldname] || 'undefined' === typeof row[fieldname] || '' === row[fieldname]) {
+            obj[camelCase(fieldname)] = row[fieldname] || obj[camelCase(fieldname)] || row[fieldname];
+          }
+          else {
             obj[camelCase(fieldname)] = row[fieldname];
+          }
         });
 
         return obj;