避免 MongoDB 中出现重复条目​​？

db.yourCollectionName.createIndex({"yourFieldName":1},{unique:true});

> db.avoidDuplicateEntriesDemo.createIndex({"UserName":1},{unique:true});
{
   "createdCollectionAutomatically" : true,
   "numIndexesBefore" : 1,
   "numIndexesAfter" : 2,
   "ok" : 1
}

> db.avoidDuplicateEntriesDemo.insertOne({"UserName":"John"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5c90e1824afe5c1d2279d697")
}

> db.avoidDuplicateEntriesDemo.insertOne({"UserName":"John"});
2019-03-19T18:03:08.465+0530 E QUERY [js] WriteError: E11000 duplicate key error collection: test.avoidDuplicateEntriesDemo index: UserName_1 dup key: { : "John" } :
WriteError({
   "index" : 0,
   "code" : 11000,
   "errmsg" : "E11000 duplicate key error collection: test.avoidDuplicateEntriesDemo index: UserName_1 dup key: { : \"John\" }",
   "op" : {
      "_id" : ObjectId("5c90e1844afe5c1d2279d698"),
      "UserName" : "John"
   }
})
WriteError@src/mongo/shell/bulk_api.js:461:48
Bulk/mergeBatchResults@src/mongo/shell/bulk_api.js:841:49
Bulk/executeBatch@src/mongo/shell/bulk_api.js:906:13
Bulk/this.execute@src/mongo/shell/bulk_api.js:1150:21
DBCollection.prototype.insertOne@src/mongo/shell/crud_api.js:252:9
@(shell):1:1

> db.avoidDuplicateEntriesDemo.insertOne({"UserName":"Carol"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5c90e18d4afe5c1d2279d699")
}

> db.avoidDuplicateEntriesDemo.find();

{ "_id" : ObjectId("5c90e1824afe5c1d2279d697"), "UserName" : "John" }
{ "_id" : ObjectId("5c90e18d4afe5c1d2279d699"), "UserName" : "Carol" }