fix(document): make toObject() use child schema flattenMaps optio…

…n by default

Fix #9995

lib/document.js

@@ -3202,10 +3202,6 @@ Document.prototype.$toObject = function(options, json) {
   options = utils.isPOJO(options) ? clone(options) : {};
   options._calledWithOptions = options._calledWithOptions || clone(options);
 
-  if (!('flattenMaps' in options)) {
-    options.flattenMaps = defaultOptions.flattenMaps;
-  }
-
   let _minimize;
   if (options._calledWithOptions.minimize != null) {
     _minimize = options.minimize;
@@ -3215,14 +3211,24 @@ Document.prototype.$toObject = function(options, json) {
     _minimize = schemaOptions.minimize;
   }
 
+  let flattenMaps;
+  if (options._calledWithOptions.flattenMaps != null) {
+    flattenMaps = options.flattenMaps;
+  } else if (defaultOptions.flattenMaps != null) {
+    flattenMaps = defaultOptions.flattenMaps;
+  } else {
+    flattenMaps = schemaOptions.flattenMaps;
+  }
+
   // The original options that will be passed to `clone()`. Important because
   // `clone()` will recursively call `$toObject()` on embedded docs, so we
   // need the original options the user passed in, plus `_isNested` and
   // `_parentOptions` for checking whether we need to depopulate.
   const cloneOptions = Object.assign(utils.clone(options), {
     _isNested: true,
     json: json,
-    minimize: _minimize
+    minimize: _minimize,
+    flattenMaps: flattenMaps
   });
 
   if (utils.hasUserDefinedProperty(options, 'getters')) {