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For transactions:对于事务：
The collections used in a transaction can be in different databases.事务中使用的集合可以位于不同的数据库中。
You cannot create new collections in cross-shard write transactions. 不能在跨分片写入事务中创建新集合。For example, if you write to an existing collection in one shard and implicitly create a collection in a different shard, MongoDB cannot perform both operations in the same transaction.例如，如果在一个shard中写入现有集合，并在另一个shard中隐式创建集合，则MongoDB无法在同一事务中执行这两个操作。
"snapshot" when reading from a capped collection. (Starting in MongoDB 5.0)"snapshot"。（从MongoDB 5.0开始）config, admin, or local databases.config、admin或local数据库中的集合。system.* collections.system.*集合。explain).explain）。getMore inside the transaction.getMore。getMore outside the transaction.getMore。killCursors as the first operation in a transaction.killCursors指定为事务中的第一个操作。Operations that affect the database catalog, such as creating or dropping a collection or an index, are not allowed in multi-document transactions. 在多文档事务中，不允许执行影响数据库目录的操作，例如创建或删除集合或索引。For example, a multi-document transaction cannot include an insert operation that would result in the creation of a new collection. 例如，多文档事务不能包含导致创建新集合的插入操作。See Restricted Operations.请参阅受限操作。
The following read/write operations are allowed in transactions:事务中允许以下读/写操作：
db.collection.aggregate() | aggregate |
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db.collection.countDocuments() |
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db.collection.distinct() | distinct |
$group stage. $group阶段。 |
db.collection.find() | find | |
delete | ||
findAndModify |
| |
insert |
| |
update |
| |
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Starting in MongoDB 4.2, you can update a document's shard key value (unless the shard key field is the immutable 从MongoDB 4.2开始，您可以通过在事务或可重试写入中发出单个文档更新/查找和修改操作来更新文档的shard键值（除非分片键字段是不可变的_id field) by issuing single-document update/findAndModify operations either in a transaction or as a retryable write. _id字段）。For details, see Change a Document's Shard Key Value.有关详细信息，请参阅更改文档的Shard键值。
To perform a count operation within a transaction, use the 要在事务中执行计数操作，请使用$count aggregation stage or the $group (with a $sum expression) aggregation stage.$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 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) as a helper method that uses the $group with a $sum expression to perform a count. countDocuments(filter, options)作为辅助方法，使用带有$sum表达式的$group来执行计数。The 4.0 drivers have deprecated the 4.0驱动程序已经弃用了count() API.count()API。
Starting in MongoDB 4.0.3, 从MongoDB 4.0.3开始，mongosh provides the db.collection.countDocuments() helper method that uses the $group with a $sum expression to perform a count.mongosh提供了db.collection.countDocuments()辅助方法，该方法使用带有$sum表达式的$group来执行计数。
To perform a distinct operation within a transaction:要在事务中执行不同的操作：
db.collection.distinct() method/the distinct command as well as the aggregation pipeline with the $group stage.db.collection.distinct()方法/distinct命令以及带有$group阶段的聚合管道。For sharded collections, you cannot use the 对于分片集合，不能使用db.collection.distinct() method or the distinct command.db.collection.distinct()方法或distinct命令。
To find the distinct values for a sharded collection, use the aggregation pipeline with the 要查找分片集合的不同值，请将聚合管道与$group stage instead. $group阶段一起使用。For example:例如：
Instead of db.coll.distinct("x"), use
db.coll.aggregate([
{ $group: { _id: null, distinctValues: { $addToSet: "$x" } } },
{ $project: { _id: 0 } }
])
Instead of 不要使用db.coll.distinct("x", { status: "A" }), use:db.coll.distinct("x", { status: "A" })，请使用：
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。
Starting in MongoDB 4.4 with feature compatibility version (fcv) 从功能兼容版本（fcv）"4.4", you can create collections and indexes inside a multi-document transaction if the transaction is not a cross-shard write transaction."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内创建集合和索引。
| Command | Notes | |
|---|---|---|
create | db.createCollection() | |
createIndexes |
For more information on creating collections and indexes in a transaction, see Create Collections and Indexes In a Transaction.有关在事务中创建集合和索引的更多信息，请参阅在事务中创建集合和索引。
You can also implicitly create a collection through the following write operations against a non-existing collection:您还可以对不存在的集合通过以下写入操作隐式创建集合：
findAndModifyupsert: true | |
insert | |
updateupsert: true | |
For other CRUD operations allowed in transactions, see CRUD Operations.有关事务中允许的其他CRUD操作，请参阅CRUD操作。
For more information on creating collections and indexes in a transaction, see Create Collections and Indexes In a Transaction.有关在事务中创建集合和索引的更多信息，请参阅在事务中创建集合和索引。
Informational commands, such as 事务中允许使用信息命令，如hello, buildInfo, connectionStatus (and their helper methods) are allowed in transactions; however, they cannot be the first operation in the transaction.hello、buildInfo、connectionStatus（及其助手方法）；但是，它们不能是事务中的第一个操作。
Changed in version 4.4.在版本4.4中更改。
The following operations are not allowed in transactions:事务中不允许以下操作：
db.createCollection() method, and indexes, e.g. db.collection.createIndexes() and db.collection.createIndex() methods, when using a read concern level other than "local".db.createCollection()方法，以及索引，例如db.collection.createIndexes()和db.collection.createIndex()方法，当使用非"local"的读取关注级别时。listCollections and listIndexes commands and their helper methods.listCollections和listIndexes命令及其助手方法。createUser, getParameter, count, etc. and their helpers.createUser、getParameter、count等及其助手。