Add custom string index support

lib/metadata.js

@@ -1,6 +1,7 @@
 'use strict';
 
 const { getKindMetadata } = require('./kinds.js');
+const { isStringIndex } = require('./util.js');
 
 const errPrefix = 'Field';
 const OPTIONS = ['validate', 'parse', 'serialize', 'format'];
@@ -61,7 +62,8 @@ class Options {
 class Indexes {
   extract(key, field) {
     const { index, primary, unique, many } = field;
-    const isIndex = Array.isArray(index || primary || unique);
+    const chosenIndex = index || primary || unique;
+    const isIndex = Array.isArray(chosenIndex) || isStringIndex(chosenIndex);
     if (isIndex || many) this[key] = field;
     return isIndex;
   }

lib/util.js

@@ -1,6 +1,6 @@
 'use strict';
 
-const { isFirstLetter } = require('metautil');
+const { isFirstLetter, validateBracketBalance } = require('metautil');
 
 const formatters = {
   type: (type, req = true) => {
@@ -41,9 +41,22 @@ const firstKey = (obj) => Object.keys(obj).find(isFirstLetter);
 
 const isInstanceOf = (obj, constrName) => obj?.constructor?.name === constrName;
 
+const isStringIndex = (index) => {
+  if (typeof index !== 'string') return false;
+  const brackets = ['(', ')'];
+  if (brackets.some((bracket) => index.indexOf(bracket) === -1)) return false;
+  if (!validateBracketBalance(index, brackets.join(''))) return false;
+  const expression = index.substring(
+    index.indexOf(brackets[0]) + 1,
+    index.lastIndexOf(brackets[1]),
+  );
+  return expression.trim().length !== 0;
+};
+
 module.exports = {
   formatters,
   checks,
   firstKey,
   isInstanceOf,
+  isStringIndex,
 };