MongoDB 中的 find() 和 findOne() 方法显示不同的结果 [英] find() and findOne() methods in MongoDB showing different results

问题描述

我有一个 Mongo 数据库，在用户集合中我只有 1 个文档.我使用用户名过滤器执行 find() 和 findOne() 操作.我从 find() 操作中得到了我认为不正确的结果.

MongoDB shell 版本:3.2.10连接到:测试服务器有启动警告:2016-10-20T20:37:32.681-0700 我控制 [initandlisten]2016-10-20T20:37:32.681-0700 我控制 [initandlisten] ** 警告:/sys/kernel/mm/transparent_hugepage/enabled 是总是".2016-10-20T20:37:32.681-0700 I CONTROL [initandlisten] ** 我们建议将其设置为从不"2016-10-20T20:37:32.681-0700 我控制 [initandlisten]2016-10-20T20:37:32.681-0700 我控制 [initandlisten] ** 警告:/sys/kernel/mm/transparent_hugepage/defrag 是总是".2016-10-20T20:37:32.681-0700 I CONTROL [initandlisten] ** 我们建议将其设置为从不"2016-10-20T20:37:32.681-0700 我控制 [initandlisten]>使用实验室2切换到 db lab2>db.users.find(){_id":ObjectId(5807ac0765f24dd0660e4332")，用户名":avtrulzz"，fname":Abc"，lname":Def"，电子邮件":[e-mail]"，密码":rootuser"，手机":NumberLong(1234567890) }>db.users.findOne(){"_id" : ObjectId("5807ac0765f24dd0660e4332"),用户名":avtulzz"，"fname": "ABC","lname" : "定义","email" : "[e-mail]",密码":rootuser"，手机":NumberLong(1234567890)}>如果(db.users.find({用户名:noSuchUsername"})){...打印(用户名存在")；... } 别的 {...打印(用户不存在")；}用户名存在>if (db.users.findOne({username : "noSuchUsername"})) { print ("用户名存在");} else { 打印(用户不存在")；}用户不存在>if (db.users.findOne({username : "avtulzz"})) { print ("用户名存在");} else { 打印(用户不存在")；}用户名存在

<块引用>

看到 find() 操作返回用户存在，这是不正确的.findOne() 运行正常.

解决方案

首先，findOne()和find()的基本区别:

• findOne() - 如果查询匹配，则返回第一个文档，否则返回 null.

• find() - 无论匹配多少个文档，都会返回一个游标，永远不会为空.

因此，当放入 if 条件时，findOne() 可以在不匹配任何文档时转换为 false.由于 find() 返回一个游标对象并且从不返回 null，因此在放入 if 条件时将转换为 true.

find 和 findOne() 为空集合返回以下内容:

I have a Mongo database where in the users collection I have just 1 document. I do a find() and a findOne() operations using the username filter. I get what I think is an incorrect result from find() operation.

MongoDB shell version: 3.2.10
connecting to: test
Server has startup warnings: 
2016-10-20T20:37:32.681-0700 I CONTROL  [initandlisten] 
2016-10-20T20:37:32.681-0700 I CONTROL  [initandlisten] ** WARNING: /sys/kernel/mm/transparent_hugepage/enabled is 'always'.
2016-10-20T20:37:32.681-0700 I CONTROL  [initandlisten] **        We suggest setting it to 'never'
2016-10-20T20:37:32.681-0700 I CONTROL  [initandlisten] 
2016-10-20T20:37:32.681-0700 I CONTROL  [initandlisten] ** WARNING: /sys/kernel/mm/transparent_hugepage/defrag is 'always'.
2016-10-20T20:37:32.681-0700 I CONTROL  [initandlisten] **        We suggest setting it to 'never'
2016-10-20T20:37:32.681-0700 I CONTROL  [initandlisten] 
> use lab2
switched to db lab2
> db.users.find()
{ "_id" : ObjectId("5807ac0765f24dd0660e4332"), "username" : "avtrulzz", "fname" : "Abc", "lname" : "Def", "email" : "[e-mail]", "password" : "rootuser", "mobile" : NumberLong(1234567890) }
> db.users.findOne()
{
    "_id" : ObjectId("5807ac0765f24dd0660e4332"),
    "username" : "avtrulzz",
    "fname" : "Abc",
    "lname" : "Def",
    "email" : "[e-mail]",
    "password" : "rootuser",
    "mobile" : NumberLong(1234567890)
}
> if (db.users.find({username : "noSuchUsername"})) {
... print ("Username exists"); 
... } else {
... print ("User does not exist"); }
Username exists
> if (db.users.findOne({username : "noSuchUsername"})) { print ("Username exists");  } else { print ("User does not exist"); }
User does not exist
> if (db.users.findOne({username : "avtrulzz"})) { print ("Username exists");  } else { print ("User does not exist"); }
Username exists

See the find() operation is returning user exists which is not true. findOne() is behaving correctly.

解决方案

First of all, basic difference between findOne() and find():

• findOne() - if query matches, first document is returned, otherwise null.

• find() - no matter the number of documents matched, a cursor is returned, never null.

So when put in an if condition, findOne() can convert to false when it doesn't match any document. As find() returns a cursor object and never returns null, will convert to true when put in an if condition.

find and findOne() return the following for empty collection :

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