November 2021
Intermediate to advanced
410 pages
12h 1m
Chinese
Content preview from MongoDB权威指南（第3版）
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查询
｜
55
这时，不能直接使用
db.blog.find({"comments" : {"author" : "joe", "score" : {"$gte" :
5}}})
进行查询。内嵌文档的匹配必须匹配整个文档，而这个查询不会匹配
"comment"
键。
使用
db.blog.find({"comments.author" : "joe", "comments.score" : {"$gte" : 5}})
也
不行，因为符合作者条件的评论与符合分数条件的评论可能不是同一条。也就是说，这会
返回上面显示的那个文档：因为它匹配了第一条评论中的
"author" : "joe"
和第二条评论
中的
"score" : 6
。
要正确指定一组条件而无须指定每个键，请使用
"$elemMatch"
。这种模糊的命名条件允许
你在查询条件中部分指定匹配数组中的单个内嵌文档。正确的查询如下所示：
> db.blog.find({"comments" : {"$elemMatch" :
... {"author" : "joe", "score" : {"$gte" : 5}}}})
"$elemMatch"
允许你将限定条件进行“分组”。仅当需要对一个内嵌文档的多个键进行操
作时才会用到它。
4.4
$where
查询
键
–
值对是一种相当有表现力的查询方式，但有些查询依然无法表示。对于无法以其他方
式执行的查询，可以使用
"$where"
子句，它允许你在查询中执行任意的
JavaScript
代码。
这样就能在查询中做大部分事情了。为安全起见，应该严格限制或消除 ...
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I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.Amir M.
I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.