Convert an Existing Index to a Unique Index将现有索引转换为唯一索引

Populate sample data填充样本数据

Create the apples collection:创建apples集合：

db.apples.insertMany( [
   { type: "Delicious", quantity: 12 },
   { type: "Macintosh", quantity: 13 },
   { type: "Delicious", quantity: 13 },
   { type: "Fuji", quantity: 15 },
   { type: "Washington", quantity: 10 }
] )

Create a single field index创建单个字段索引

Add a single field index on the type field:在type字段上添加单个字段索引：

db.apples.createIndex( { type: 1 } )

Prepare the index to be converted to a unique index准备要转换为唯一索引的索引

Run collMod on the type field index and set prepareUnique to true:在type字段索引上运行collMod，并将prepareUnique设置为true：

db.runCommand( {
   collMod: "apples",
   index: {
      keyPattern: { type: 1 },
      prepareUnique: true
   }
} )

After prepareUnique is set, you cannot insert new documents that duplicate an index key entry. For example, the following insert operation results in an error:设置prepareUnique后，您将无法插入重复索引键条目的新文档。例如，以下插入操作会导致错误：

db.apples.insertOne( { type: "Delicious", quantity: 20 } )

MongoServerError: E11000 duplicate key error collection:
test.apples index: type_1 dup key: { type: "Delicious" }

Check for unique key violations检查是否存在唯一键违规

To see if there are any documents that violate the unique constraint on the type field, run collMod with unique: true and dryRun: true:要查看是否有任何文档违反了类型字段的唯一约束，请使用unique:true和dryRun:true运行collMod：

db.runCommand( {
   collMod: "apples",
   index: {
      keyPattern: { type: 1 },
      unique: true
   },
   dryRun: true
} )

MongoServerError: Cannot convert the index to unique. Please resolve conflicting documents before running collMod again.

Violations: [
   {
      ids: [
         ObjectId("660489d24cabd75abebadbd0"),
         ObjectId("660489d24cabd75abebadbd2")
      ]
   }
]

Resolve duplicate key conflicts解决重复键冲突

To complete the conversion, modify the duplicate entries to remove any conflicts. For example:要完成转换，请修改重复条目以消除任何冲突。例如：

db.apples.deleteOne(
   { _id: ObjectId("660489d24cabd75abebadbd2") }
)

Confirm that all conflicts are resolved确认所有冲突均已解决

To confirm that the index can be converted, re-run the collMod() command with dryRun: true:要确认索引可以转换，请使用dryRun:true重新运行collMod()命令：

db.runCommand( {
   collMod: "apples",
   index: {
      keyPattern: { type: 1 },
      unique: true
   },
   dryRun: true
} )

{ ok: 1 }

Finalize the index conversion完成索引转换

To finalize the conversion to a unique index, run the collMod command with unique: true and remove the dryRun flag:要完成向唯一索引的转换，请使用unique:true运行collMod命令并删除dryRun标志：

db.runCommand( {
   collMod: "apples",
   index: {
      keyPattern: { type: 1 },
      unique: true
   }
} )

{ unique_new: true, ok: 1 }