fix(model): fix syncIndexes() error when db index has a collation b…

…ut Mongoose index does not

Fix #9224

lib/helpers/indexes/isIndexEqual.js

@@ -3,6 +3,16 @@
 const get = require('../get');
 const utils = require('../../utils');
 
+/**
+ * Given a Mongoose index definition (key + options objects) and a MongoDB server
+ * index definition, determine if the two indexes are equal.
+ *
+ * @param {Object} key the Mongoose index spec
+ * @param {Object} options the Mongoose index definition's options
+ * @param {Object} dbIndex the index in MongoDB as returned by `listIndexes()`
+ * @api private
+ */
+
 module.exports = function isIndexEqual(key, options, dbIndex) {
   // If these options are different, need to rebuild the index
   const optionKeys = [
@@ -17,8 +27,8 @@ module.exports = function isIndexEqual(key, options, dbIndex) {
       continue;
     }
     if (key === 'collation') {
-      if (!(key in options) || !(key in dbIndex)) {
-        return false;
+      if (options.key == null || dbIndex.key == null) {
+        return options.key == null && dbIndex.key == null;
       }
       const definedKeys = Object.keys(options.collation);
       const schemaCollation = options.collation;