测试数据：

{ "_id" : ObjectId("4b97e62bf1d8c7152c9ccb74"), "title" : 
"ABC", 
  "comments" : [ { "by" : "joe", "votes" : 3 }, { "by" : "jane", 
"votes" : 7 } ] } 

执行更新语句：

语句一：

db.mydb.update({title:"ABC"}, {$inc:{"comments.$.votes":1}}, 0, 1)

执行报错：Cannot apply the positional operator without a corresponding query field containing an array.

语句二：

db.mydb.update({"comments.by":"joe"}, {$inc:{"comments.$.votes":1}}, 0, 1)

执行结果正常

请问是怎么回事？