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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166
｜
第
7
章
7.9
将聚合管道结果写入集合中
作为两个特定的阶段，
$out
和
$merge
可以将聚合管道生成的文档写入集合中。这两个阶段
不能同时使用，并且任一阶段必须是聚合管道的最后一个阶段。
$merge
是在
MongoDB 4.2
中引入的，如果可以使用的话，它是将结果写入集合中的首选方式。
$out
有一些限制：它
只能写入相同的数据库；如果集合已经存在，那么它会覆盖任何现有的集合；它不能写入
已分片的集合中。
$merge
可以写入任何数据库和集合中，无论是否分片。
$merge
还可以在
处理现有集合时对结果进行合并（插入新文档、与现有文档合并、操作失败、保留现有文
档或使用自定义更新处理所有文档）。但使用
$merge
的真正优势在于，它可以创建按需生
成的物化视图（
materialized
view
），在管道运行的过程中，输出到集合的内容会进行增量
更新。
本章讨论了许多累加器，其中一些可以在投射阶段使用，还讨论了在使用各种累加器时是
使用分组还是投射。接下来看看
MongoDB
中的事务。
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