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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聚合框架
｜
151
{ $match: {"funding_rounds.investments.financial_org.permalink": "greylock"} },
{ $project: {
_id: 0,
name: 1,
individualFunder: "$funding_rounds.investments.person.permalink",
fundingOrganization: "$funding_rounds.investments.financial_org.permalink",
amount: "$funding_rounds.raised_amount",
year: "$funding_rounds.funded_year"
} }
])
这个管道将首先过滤出
Greylock
至少参与过一轮融资的公司，然后展开融资轮并再次进行
过滤，这样就只有
Greylock
实际参与融资轮的文档才会被传递到投射阶段。
正如本章开头提到的，常常需要包含相同类型的多个阶段。这是一个很好的例子：通过过
滤来减少最初查看的文档数量，将文档集缩小到
Greylock
至少参与一轮融资的那些文档
。
然后，在展开阶段最终输出一些代表
Greylock
所投资公司的融资轮的文档
，但实际上有个
别的融资轮
Greylock
并没有参与
。可以简单地加入另一个过滤器，使用第二个匹配阶段，
来排除所有不感兴趣的融资轮。
7.6
数组表达式
现在把注意力转向数组表达式。我们会尝试在投射阶段中使用数组表达式，它是本书将深 ...
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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.