code: function(fieldName) {
      var field;
      if (this.domain && fieldName) {
        field = this.domain.getIndexField(fieldName);
        if (!field.exists()) {
          this.throwValidationError(
            'CS-UnknownFieldInMatchExpression',
            'Field \'' + fieldName + '\' is not defined in the metadata for this ' +
              'collection. All fields used in the match expression must be ' +
              'defined in the metadata.'
          );
        } else if (!field.searchEnabled) {
          this.available = false;
        }
      }
      if (!field)
        field = new IndexField(fieldName).setType("text");
      return field;
    }
docs: Get an instance of "IndexField". Unknown field works as "text, searchable" field, if it works without domain.