fix(document): dont leave nested key as undefined when setting nested…

… key to empty object with minimize

Fix #8565

lib/document.js

@@ -893,7 +893,11 @@ Document.prototype.$set = function $set(path, val, type, options) {
       keys = Object.keys(path);
       const len = keys.length;
 
-      if (len === 0 && !this.schema.options.minimize) {
+      // `_skipMinimizeTopLevel` is because we may have deleted the top-level
+      // nested key to ensure key order.
+      const _skipMinimizeTopLevel = get(options, '_skipMinimizeTopLevel', false);
+      if (len === 0 && (!this.schema.options.minimize || _skipMinimizeTopLevel)) {
+        delete options._skipMinimizeTopLevel;
         if (val) {
           this.$set(val, {});
         }
@@ -924,6 +928,8 @@ Document.prototype.$set = function $set(path, val, type, options) {
         this._doc[key] != null &&
         Object.keys(this._doc[key]).length === 0) {
       delete this._doc[key];
+      // Make sure we set `{}` back even if we minimize re: gh-8565
+      options = Object.assign({}, options, { _skipMinimizeTopLevel: true });
     }
 
     if (typeof path[key] === 'object' &&