code: function(value, attr, required, model, computed) {
          var isRequired = _.isFunction(required) ? required.call(model, value, attr, computed) : required;
          if(!isRequired && !hasValue(value)) {
            return false; // overrides all other validators
          }
          if (isRequired && !hasValue(value)) {
            return this.format(getMessageKey(this.msgKey, defaultMessages.required), this.formatLabel(attr, model));
          }
        }
docs: Required validator Validates if the attribute is required or not This can be specified as either a boolean value or a function that returns a boolean value