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
｜
145
在投射阶段中，用于指定
ipo
、
valuation
和
funders
值的
$
字符表示这些值应被解释为字
段路径，并分别用于为每个字段选择应投射的值。
你可能已经注意到
funders
显示出了多个值。实际上，我们看到的是数组的数组。根据
Facebook
示例文档
，所有的投资方都被列在一个名为
"investments"
的数组中。阶段中
指定了对于每一轮融资，都要为
"investments"
数组中的每个条目投射
financial_org.
permalink
值。因此，一个由投资方名字组成的数组就建立起来了。
本章稍后会介绍如何对字符串、日期和许多其他值类型执行算术运算和其他操作，以投射
各种形状和大小的文档。在投射阶段，唯一不能做的事情就是更改值的数据类型。
7.5
$unwind
在聚合管道中处理数组字段时，通常需要包含一个或多个展开（
unwind
）阶段。这允许我
们将指定数组字段中的每个元素都形成一个输出文档，如图
7-4
所示。
{ key1: "value1",
key2: "value2",
key3: ["elem1",
"elem2",
"elem3"]}
{ key1: "value1",
key2: "value2",
key3: "elem1"}
{ key1: "value1",
key2: "value2",
key3: "elem2"}
{ key1: "value1",
key2: "value2",
key3: "elem3"}
$unwind
图 7-4：
$unwind ...
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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.