Model Data for Atomic Operations原子操作的模型数据

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Although MongoDB supports multi-document transactions for replica sets (starting in version 4.0) and sharded clusters (starting in version 4.2), for many scenarios, the denormalized data model, as discussed on this page, will continue to be optimal for your data and use cases.尽管MongoDB支持副本集（从版本4.0开始）和分片集群（从版本4.2开始）的多文档事务，但对于许多场景，如本页所述，非规范化数据模型将继续适合您的数据和用例。

Pattern图案

In MongoDB, a write operation on a single document is atomic. 在MongoDB中，对单个文档的写入操作是原子的。For fields that must be updated together, embedding the fields within the same document ensures that the fields can be updated atomically.对于必须一起更新的字段，在同一文档中嵌入这些字段可以确保这些字段可以进行原子更新。

For example, consider a situation where you need to maintain information on books, including the number of copies available for checkout as well as the current checkout information.例如，考虑一个需要维护图书信息的情况，包括可用于结帐的副本数量以及当前结帐信息。

The available copies of the book and the checkout information should be in sync. 书的可用副本和结帐信息应同步。As such, embedding the available field and the checkout field within the same document ensures that you can update the two fields atomically.因此，在同一文档中嵌入available字段和checkout字段可以确保可以原子地更新这两个字段。

{
    _id: 123456789,
    title: "MongoDB: The Definitive Guide",
    author: [ "Kristina Chodorow", "Mike Dirolf" ],
    published_date: ISODate("2010-09-24"),
    pages: 216,
    language: "English",
    publisher_id: "oreilly",
available: 3,checkout: [ { by: "joe", date: ISODate("2012-10-15") } ]}

Then to update with new checkout information, you can use the db.collection.updateOne() method to atomically update both the available field and the checkout field:然后，要使用新的签出信息进行更新，可以使用db.collection.updateOne()方法以原子方式更新可用字段和签出字段：

db.books.updateOne (
   { _id: 123456789, available: { $gt: 0 } },
   {
     $inc: { available: -1 },
     $push: { checkout: { by: "abc", date: new Date() } }
   }
)

The operation returns a document that contains information on the status of the operation:该操作返回一个文档，其中包含有关操作状态的信息：

{ "acknowledged" : true, "matchedCount" : 1, "modifiedCount" : 1 }

The matchedCount field shows that 1 document matched the update condition, and modifiedCount shows that the operation updated 1 document.matchedCount字段显示1个文档符合更新条件，modifiedCount显示操作更新了1个文档。

If no document matched the update condition, then matchedCount and modifiedCount would be 0 and would indicate that you could not check out the book.如果没有与更新条件匹配的文档，则matchedCount和modifiedCount将为0，并表示您无法签出该书。