fix(cursor): throw error when using aggregation cursor as async iterator

Fix #8280

lib/cursor/AggregationCursor.js

@@ -4,6 +4,7 @@
 
 'use strict';
 
+const MongooseError = require('../error/mongooseError');
 const Readable = require('stream').Readable;
 const eachAsync = require('../helpers/cursor/eachAsync');
 const util = require('util');
@@ -94,6 +95,15 @@ AggregationCursor.prototype._read = function() {
   });
 };
 
+if (Symbol.asyncIterator != null) {
+  const msg = 'Mongoose does not support using async iterators with an ' +
+    'existing aggregation cursor. See http://bit.ly/mongoose-async-iterate-aggregation';
+
+  AggregationCursor.prototype[Symbol.asyncIterator] = function() {
+    throw new MongooseError(msg);
+  };
+}
+
 /**
  * Registers a transform function which subsequently maps documents retrieved
  * via the streams interface or `.next()`