message: default to previous class names if not provided
diff: @@ -140,7 +140,7 @@ export default class Field {
  this.name = options.name || this.name || null;
  this.rules = options.rules ? normalizeRules(options.rules) : this.rules;
  this.model = options.model || this.model;
  - this.classNames = options.classNames;
  + this.classNames = options.classNames || this.classNames;
  this.expression = JSON.stringify(options.expression);
  this.alias = options.alias;
  this.getter = isCallable(options.getter) ? options.getter : this.getter;
  