Changes

node_modules/mongoose/lib/helpers/document/compile.js

@@ -124,6 +124,13 @@ function defineKey(prop, subprops, prototype, prefix, keys, options) {
             }
           });
 
+          Object.defineProperty(nested, '$__parent', {
+            enumerable: false,
+            configurable: true,
+            writable: false,
+            value: this
+          });
+
           compile(subprops, nested, path, options);
           this.$__.getters[path] = nested;
         }
@@ -131,8 +138,10 @@ function defineKey(prop, subprops, prototype, prefix, keys, options) {
         return this.$__.getters[path];
       },
       set: function(v) {
-        if (v instanceof Document) {
-          v = v.toObject({ transform: false });
+        if (v != null && v.$__isNested) {
+          // Convert top-level to POJO, but leave subdocs hydrated so `$set`
+          // can handle them. See gh-9293.
+          v = v.$__parent.get(path);
         }
         const doc = this.$__[scopeSymbol] || this;
         doc.$set(path, v);