Transactions and Operations事务和操作

CRUD OperationsCRUD操作

The following read/write operations are allowed in transactions:事务中允许以下读/写操作：

Count Operation计数操作

To perform a count operation within a transaction, use the $count aggregation stage or the $group (with a $sum expression) aggregation stage.要在事务中执行计数操作，请使用$count聚合阶段或$group（带有$sum表达式）聚合阶段。

MongoDB drivers compatible with the 4.0 features provide a collection-level API countDocuments(filter, options) as a helper method that uses the $group with a $sum expression to perform a count. The 4.0 drivers have deprecated the count() API.与4.0功能兼容的MongoDB驱动程序提供了一个集合级API countDocuments(filter, options)作为一个助手方法，使用带有$sum表达式的$group来执行计数。4.0驱动程序已弃用count()API。

Starting in MongoDB 4.0.3, mongosh provides the db.collection.countDocuments() helper method that uses the $group with a $sum expression to perform a count.从MongoDB 4.0.3开始，mongosh提供了db.collection.countDocuments()辅助方法，该方法使用带有$sum表达式的$group来执行计数。

Distinct Operation独特的操作

To perform a distinct operation within a transaction:要在事务中执行不同的操作：

• For unsharded collections, you can use the db.collection.distinct() method/the distinct command as well as the aggregation pipeline with the $group stage.对于未排序的集合，可以使用db.collection.distinct()方法/distinct命令以及带有$group阶段的聚合管道。
• For sharded collections, you cannot use the db.collection.distinct() method or the distinct command.对于分片集合，不能使用db.collection.distinct()方法或distinct命令。

  To find the distinct values for a sharded collection, use the aggregation pipeline with the $group stage instead. 要查找分片集合的不同值，请使用带有$group阶段的聚合管道。For example:例如：

  • Instead of db.coll.distinct("x"), use不使用db.coll.distinct("x")，而是使用

    db.coll.aggregate([
       { $group: { _id: null, distinctValues: { $addToSet: "$x" } } },
       { $project: { _id: 0 } }
    ])

  • Instead of db.coll.distinct("x", { status: "A" }), use:不使用db.coll.distinct("x", { status: "A" })，而是使用：

    db.coll.aggregate([
       { $match: { status: "A" } },
       { $group: { _id: null, distinctValues: { $addToSet: "$x" } } },
       { $project: { _id: 0 } }
    ])

  The pipeline returns a cursor to a document:管道将游标返回到文档：

  { "distinctValues" : [ 2, 3, 1 ] }

  Iterate the cursor to access the results document.迭代游标以访问结果文档。

DDL OperationsDDL操作

Starting in MongoDB 4.4 with feature compatibility version (fcv) "4.4", you can create collections and indexes inside a multi-document transaction if the transaction is not a cross-shard write transaction.从功能兼容版本（fcv）“4.4”的MongoDB 4.4开始，如果事务不是跨分片写入事务，则可以在多文档事务中创建集合和索引。

Informational Operations信息操作

Informational commands, such as hello, buildInfo, connectionStatus (and their helper methods) are allowed in transactions; however, they cannot be the first operation in the transaction.事务中允许使用信息命令，如hello、buildInfo、connectionStatus（及其辅助方法）；但是，它们不能是事务中的第一个操作。