如何限制在MongoDB中插入相同名称的项目？
为此，使用ensureIndex()并设置unique：true。让我们创建包含文档的集合。在这里，当我们尝试惰性化重复项时，则发生重复键错误-
> db.demo316.ensureIndex({"SubjectName":1},{unique:true});
{
"createdCollectionAutomatically" : true,
"numIndexesBefore" : 1,
"numIndexesAfter" : 2,
"ok" : 1
}
> db.demo316.insertOne({"SubjectName":"MySQL"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e50e378f8647eb59e56205d")
}
> db.demo316.insertOne({"SubjectName":"MongoDB"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e50e37df8647eb59e56205e")
}
> db.demo316.insertOne({"SubjectName":"MongoDB"});
2020-02-22T13:47:05.186+0530 E QUERY [js] WriteError: E11000 duplicate key error collection: test.demo316 index: SubjectName_1 dup key: { : "MongoDB" } :
WriteError({
"index" : 0,
"code" : 11000,
"errmsg" : "E11000 duplicate key error collection: test.demo316 index: SubjectName_1 dup key: { : \"MongoDB\" }",
"op" : {
"_id" : ObjectId("5e50e381f8647eb59e56205f"),
"SubjectName" : "MongoDB"
}
})
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在find()方法的帮助下显示集合中的所有文档-
> db.demo316.find();
这将产生以下输出-
{ "_id" : ObjectId("5e50e378f8647eb59e56205d"), "SubjectName" : "MySQL" }
{ "_id" : ObjectId("5e50e37df8647eb59e56205e"), "SubjectName" : "MongoDB" }