MongoDB 数组和投影

Question

我正在学习 MongoDB，但在执行简单查询时遇到了问题。我有以下文档结构:

{
"_id" : 1,
"Title" : "Title 1",
"Author" : "Author 1",
"Comments" : [ 
    {
        "Id" : "Comment1_Id",
        "User" : "User 1",
        "Content" : "Content 1"
    }, 
    {
        "Id" : "Comment2_Id",
        "User" : "User 2",
        "Content" : "Content 2"
    }, 
    {
        "Id" : "Comment3_Id",
        "User" : "User 3",
        "Content" : "Content 3"
    }, 
    {
        "Id" : "Comment4_Id",
        "User" : "User 4",
        "Content" : "Content 4"
    }, 
    {
        "Id" : "Comment5_Id",
        "User" : "User 5",
        "Content" : "Content 5"
    }
] }

我正在练习和做一些基本的事情。我必须找到特定给定用户发布的所有内容。我尝试了以下查询:

db.articles.find({
    "Comments.User" : "User 5" },
  {
    "Comments.User" : 1, "Comments.Content" : 1 
  });

我希望

{
"_id" : 1,
"Comments" : [ 
    {
        "User" : "User 5",
        "Content" : "Content 5"
    }
] }

但是我明白了

{
"_id" : 1,
"Comments" : [ 
    {
        "User" : "User 1",
        "Content" : "Content 1"
    }, 
    {
        "User" : "User 2",
        "Content" : "Content 2"
    }, 
    {
        "User" : "User 3",
        "Content" : "Content 3"
    }, 
    {
        "User" : "User 4",
        "Content" : "Content 4"
    }, 
    {
        "User" : "User 5",
        "Content" : "Content 5"
    }
] }

我错过了什么吗？感谢您的帮助

Answer 1

Mongo 的find 返回匹配查询的整个文档，它不会只返回匹配的数组元素。

要仅返回匹配的元素，您需要进行聚合并使用展开运算符:https://docs.mongodb.com/manual/reference/operator/aggregation/unwind/