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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应用程序设计
｜
185
最简单的方式是简单地根据应用程序的需要改进数据库模式，以确保应用程序支持所有的
旧版模式（例如，接受某些字段的缺失，或者优雅地处理某个字段的多个可能的类型）。
但是这种方式可能会导致混乱，特别是当不同版本的模式存在冲突时。例如，某个版本
需要一个
"mobile"
字段，另一个版本则不需要这个字段，而是需要另外一个不同的字段，
还有一个版本会将
"mobile"
视为可选字段。跟踪这些变化的需求可能会逐渐使代码变得
一团糟。
为了以一种更结构化的方式处理不断变化的需求，可以在每个文档中包含一个
"version"
字段（或者仅仅是
"v"
），并使用它来确定应用程序将接受的文档结构。这将对模式的要求
更加严格：文档必须对某个版本有效。不过，这仍然需要支持各种旧版本。
最后一种方式是在模式变更时迁移所有数据。通常来说这不是一个好主意：
MongoDB
允
许使用动态模式来避免迁移，因为这会给系统带来很大压力。不过，如果决定更改每一个
文档，则需要确保所有文档都被成功更新。
MongoDB
的
事务
可以支持这种类型的迁移。
如果
MongoDB
在事务处理过程中崩溃，那么旧的模式将被保留。
9.7
模式管理
MongoDB 3.2
引入了模式验证，其可以在更新和插入操作期间对数据进行验证。
MongoDB 3.6
又通过
$jsonSchema
运算符添加了
JSON
模式验证，现在这是
MongoDB
中所有模式验证的
推荐方法。在撰写本书时，
MongoDB
支持
JSON
模式的第
4
版草案
，但还请参阅相关文
档以了解有关此特性的最新信息。 ...
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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.