mongodb "$" 操作符问题

Question

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？

Answer 1

 "comments" : [ { "by" : "joe", "votes" : 3 }, { "by" : "jane", 
"votes" : 7 } ]

Comments here is an array (see error message), if you want to update, you need to traverse it

Answer 2

The person above is right, comments is an array, but there is no need to traverse it. The "$" used by the poster is the solution. Just used it wrong. Let’s take a look at what $ means.

In the first two parameters of update, <query conditions> and <update operation>, if the content you query in <query conditions> is the content in the array, you can use the <update operation> Use "$" to reference elements matched in the previous query.

Your second example is querying an array. In the first example, you found title="ABC"的doc, 这里匹配查询条件的不包含array. 实际上，你可以把两个例子中的的查询条件写在一起，查询 title="..." and the doc with Joe’s message, and then added the vote of the found Joe’s message by +1.