组合字段'From'必须定义为对象内的表达式

Question

我在数据库中有一个收集消息。

为了显示集合结构，我使用这个命令。

db.messages.findOne（）

{ 
    "_id" : ObjectId("4f16fc97d1e2d32371003f02"), 
    "body" : "COURTYARD\n\nMESQUITE\n2300 HWY 67\nMESQUITE, TX 75150\ntel: 972-681-3300\nfax: 972-681-3324\n\nHotel Information: http://courtyard.com/DALCM\n\n\nARRIVAL CONFIRMATION:\n Confirmation Number:84029698\nGuests in Room: 2\nNAME: MR ERIC BASS \nGuest Phone: 7138530977\nNumber of Rooms:1\nArrive: Oct 6 2001\nDepart: Oct 7 2001\nRoom Type: ROOM - QUALITY\nGuarantee Method:\n Credit card guarantee\nCANCELLATION PERMITTED-BEFORE 1800 DAY OF ARRIVAL\n\nRATE INFORMATION:\nRate(s) Quoted in: US DOLLAR\nArrival Date: Oct 6 2001\nRoom Rate: 62.10 per night. Plus tax when applicable\nRate Program: AAA AMERICAN AUTO ASSN\n\nSPECIAL REQUEST:\n NON-SMOKING ROOM, GUARANTEED\n \n\n\nPLEASE DO NOT REPLY TO THIS EMAIL \nAny Inquiries Please call 1-800-321-2211 or your local\ninternational toll free number.\n \nConfirmation Sent: Mon Jul 30 18:19:39 2001\n\nLegal Disclaimer:\nThis confirmation notice has been transmitted to you by electronic\nmail for your convenience. Marriott's record of this confirmation\nnotice is the official record of this reservation. Subsequent\nalterations to this electronic message after its transmission\nwill be disregarded.\n\nMarriott is pleased to announce that High Speed Internet Access is\nbeing rolled out in all Marriott hotel brands around the world.\nTo learn more or to find out whether your hotel has the service\navailable, please visit Marriott.com.\n\nEarn points toward free vacations, or frequent flyer miles\nfor every stay you make! Just provide your Marriott Rewards\nmembership number at check in. Not yet a member? Join for free at\nhttps://member.marriottrewards.com/Enrollments/enroll.asp?source=MCRE\n\n", 
    "filename" : "2.", 
    "headers" : { 
     "Content-Transfer-Encoding" : "7bit", 
     "Content-Type" : "text/plain; charset=us-ascii", 
     "Date" : ISODate("2001-07-30T22:19:40Z"), 
     "From" : "reservations@marriott.com", 
     "Message-ID" : "<32788362.1075840323896.JavaMail.evans@thyme>", 
     "Mime-Version" : "1.0", 
     "Subject" : "84029698 Marriott Reservation Confirmation Number", 
     "To" : [ 
      "ebass@enron.com" 
     ], 
     "X-FileName" : "eric bass 6-25-02.PST", 
     "X-Folder" : "\\ExMerge - Bass, Eric\\Personal", 
     "X-From" : "Reservations@Marriott.com", 
     "X-Origin" : "BASS-E", 
     "X-To" : "EBASS@ENRON.COM", 
     "X-bcc" : "", 
     "X-cc" : "" 
    }, 
    "mailbox" : "bass-e", 
    "subFolder" : "personal" 
} 

我需要 '从' 对的结果，并 '向'。要做到这一点，我使用聚合命令。首先，我使用{"$unwind": "$headers.To"}来取消组合header.to中的所有电子邮件。比我使用$ group部分来分组我的结果。

我使用此查询：

db.messages.aggregate([{"$unwind": "$headers.To"}, 
    {"$group": { "_id":null, 'From': "$headers.From", 'To': "$headers.To","count":{$sum:1}}}, 
    {"$sort": {"count": -1}}, 
    {"$limit": 10}, 
    ]) 

错误消息：

assert: command failed: { 
    "ok" : 0, 
    "errmsg" : "the group aggregate field 'From' must be defined as an expression inside an object", 
    "code" : 15951 
} : aggregate failed 
_getErrorWithCode@src/mongo/shell/utils.js:25:13 
doassert@src/mongo/shell/assert.js:13:14 
assert.commandWorked@src/mongo/shell/assert.js:287:5 
DBCollection.prototype.aggregate@src/mongo/shell/collection.js:1312:5 
@(shell):1:1 

2016-11-24T20:07:50.827+0200 E QUERY [thread1] Error: command failed: { 
    "ok" : 0, 
    "errmsg" : "the group aggregate field 'From' must be defined as an expression inside an object", 
    "code" : 15951 
} : aggregate failed : 
_getErrorWithCode@src/mongo/shell/utils.js:25:13 
doassert@src/mongo/shell/assert.js:13:14 
assert.commandWorked@src/mongo/shell/assert.js:287:5 
DBCollection.prototype.aggregate@src/mongo/shell/collection.js:1312:5 
@(shell):1:1 

我做了什么错？

Answer 1

你想实现什么 - 你想组由&到假设 - 是可以做到以下group：

{"$group": { "_id": {'From': "$headers.From", 'To': "$headers.To"}, "count": {$sum:1}}}

从参考文献：

$群组

按某些指定的表达式对文档进行分组，并将每个不同分组的文档输出到下一个阶段。输出文档包含一个_id字段，该字段按键包含不同的组。输出文档还可以包含计算字段保持由$组的_id字段分组一些累加器表达式值的

累加器表达式表述如min，max（每组），sum（如你所用），数组表达式（由push和addToSet指定）等。

具体而言，错误消息指出您必须使用上述表达式之一作为Form字段的值。

如果你不喜欢From和To“挤”里面_id，以后可以消耗它，通过添加$project阶段。