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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40
｜
第
3
章
db.blog.updateOne(
{"post" : post_id },
{ $set: { "comments.$[elem].hidden" : true } },
{
arrayFilters: [ { "elem.votes": { $lte: -5 } }]
}
)
这个语句将
elem
定义为了
"comments"
数组中每个匹配元素的标识符。如果
elem
所标识的
评论的
votes
值小于或等于
-5
，那么就在
"comments"
文档中添加一个名为
"hidden"
的字
段，并将其值设置为
true
。
3.3.3
upsert
upsert
是一种特殊类型的更新。如果找不到与筛选条件相匹配的文档，则会以这个条件和
更新文档为基础来创建一个新文档；如果找到了匹配的文档，则进行正常的更新。
upsert
用起来
非常方便，有了它便不再需要“预置”集合：通常可以使用同一套代码创建和更新
文档。
下面再回到那个记录每个网页访问次数的例子。如果没有
upsert
，那么我们可能会尝试查
找
URL
并增加访问次数
，或者在
URL
不存在的情况下创建一个新文档。如果把它写成一
个
JavaScript
程序，它可能看起来像下面这样：
// 检查这个页面是否有一个文档
blog = db.analytics.findOne({url : "/blog"})
// 如果有，就将访问次数加1并进行保存
if (blog) {
blog.pageviews++;
db.analytics.save(blog); ...
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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.