什么mongodb查询将限制嵌套列表的结果

Question

我有以下文件：

{
    "username":"test",
    "comments":
    [
        {
            "commentId":"53453445",
            "message":"Hello this is a message"
        },
        {
            "commentId":"53453445",
            "message":"Hello this is a message"
        },
        {
            "commentId":"53453445",
            "message":"Hello this is a message"
        }
    ]
}

如何将评论限制为仅显示2个项目？

Answer 1

$slice运算符：

db.collection.find( { "username":"test" }, { comments: {$slice: 2 } } );