fix: fix tests and add some comments

Author: vkarpov15

lib/cursor/queryCursor.js

@@ -83,6 +83,9 @@ function QueryCursor(query) {
       // Max out the number of documents we'll populate in parallel at 5000.
       this.options._populateBatchSize = Math.min(this.options.batchSize, 5000);
     }
+    if (query._mongooseOptions._asyncIterator) {
+      this._mongooseOptions._asyncIterator = true;
+    }
 
     if (model.collection._shouldBufferCommands() && model.collection.buffer) {
       model.collection.queue.push([
@@ -410,7 +413,8 @@ QueryCursor.prototype.addCursorFlag = function(flag, value) {
  */
 
 if (Symbol.asyncIterator != null) {
-  QueryCursor.prototype[Symbol.asyncIterator] = function() {
+  QueryCursor.prototype[Symbol.asyncIterator] = function queryCursorAsyncIterator() {
+    // Set so QueryCursor knows it should transform results for async iterators into `{ value, done }` syntax
     this._mongooseOptions._asyncIterator = true;
     return this;
   };
@@ -433,7 +437,7 @@ function _next(ctx, cb) {
       return cb(err);
     }
 
-    // Handle null documents - if asyncIterator, we need `done: true`, otherwise just
+    // Handle null documents - if asyncIterator, we need to return `done: true`, otherwise just
     // skip. In either case, avoid transforms.
     if (doc === null) {
       if (ctx._mongooseOptions._asyncIterator) {
@@ -450,7 +454,8 @@ function _next(ctx, cb) {
       }, doc);
     }
 
-    // For async iterator, convert to {value, done} format
+    // This option is set in `Symbol.asyncIterator` code paths.
+    // For async iterator, we need to convert to {value, done} format
     if (ctx._mongooseOptions._asyncIterator) {
       return cb(null, { value: doc, done: false });
     }

lib/query.js

@@ -5476,8 +5476,10 @@ Query.prototype.nearSphere = function() {
  */
 
 if (Symbol.asyncIterator != null) {
-  Query.prototype[Symbol.asyncIterator] = function() {
-    return this.cursor().transformNull()._transformForAsyncIterator();
+  Query.prototype[Symbol.asyncIterator] = function queryAsyncIterator() {
+    // Set so QueryCursor knows it should transform results for async iterators into `{ value, done }` syntax
+    this._mongooseOptions._asyncIterator = true;
+    return this.cursor();
   };
 }