Hello All,

In this blog we will discuss whether MongoDB's $in clause guarantee order or not.

The $in operator selects the documents where the value of a field equals any value in the specified array. To specify an $in expression, use the following prototype:

db.collection_name.find( { fieldName: { $in: [ keywords_to_find] } } )

Below Code demonstrates how keytags collection values are saved in collection in natural order.

Find Clause-

> db.keytags.find().pretty()
{
    "__v" : 0,
    "_id" : ObjectId("56499d341af7854313c2fb8a"),
    "additionalProperties" : {
        "wifiTagDescription" : "",
        "networkType" : "WEP",
        "password" : "Ankit11",
        "ssid" : "ankit.chettri%40evontech.com"
    },
    "businessId" : ObjectId("55c337720545813123acf09f"),
    "clientId" : ObjectId("55c33660cfc3bb56068b4567"),
    "createdAt" : ISODate("2015-11-16T09:09:08.923Z"),
    "creatorAccountId" : ObjectId("55c3366672a63c6a2246af3e"),
    "dataChecksum" : "3d794b6847d0e7b2fcc355462b2e1d95",
    "deleted" : "false",
    "domain" : "evontech.com",
    "donotIndex" : false,
    "isDomainCard" : true,
    "kiTAG" : "connect#evontech.com",
    "lastIndexedOn" : null,
    "manualUpdate" : true,
    "name" : "connect",
    "relatedKeywords" : "",
    "seo" : true,
    "status" : "ACTIVE",
    "title" : "TEST",
    "type" : "Wi-Fi",
    "updatedAt" : ISODate("2014-12-17T06:17:17.461Z")
}
{
    "__v" : 0,
    "_id" : ObjectId("55c9cbffcad924c90f3a3e7e"),
    "additionalProperties" : {
        "contentImage" : "",
        "description" : "%3Cp%3Ehello%20test%3C%2Fp%3E%0A",
        "talkback" : true
    },
    "businessId" : ObjectId("55c337720545813123acf09f"),
    "clientId" : ObjectId("55c33660cfc3bb56068b4567"),
    "createdAt" : ISODate("2015-08-11T10:18:39.293Z"),
    "creatorAccountId" : ObjectId("55c3366672a63c6a2246af3e"),
    "dataChecksum" : "ff5baf67be234fcbc789570a2009c4d4",
    "deleted" : "false",
    "domain" : "evontech.com",
    "donotIndex" : false,
    "isDomainCard" : true,
    "kiTAG" : "about#evontech.com",
    "lastIndexedOn" : null,
    "manualUpdate" : true,
    "name" : "about",
    "relatedKeywords" : "",
    "seo" : true,
    "status" : "ACTIVE",
    "title" : "Test",
    "type" : "Content",
    "updatedAt" : ISODate("2015-05-18T06:11:53.687Z")
}
{
    "__v" : 0,
    "_id" : ObjectId("564ac5c0171af45e0d72354a"),
    "additionalProperties" : {
        "wifiTagDescription" : "",
        "networkType" : "WEP",
        "password" : "Ankit11",
        "ssid" : "ankit.chettri%40evontech.com"
    },
    "businessId" : ObjectId("55c337720545813123acf09f"),
    "clientId" : ObjectId("55c33660cfc3bb56068b4567"),
    "createdAt" : ISODate("2015-11-17T06:14:24.546Z"),
    "creatorAccountId" : ObjectId("55c3366672a63c6a2246af3e"),
    "dataChecksum" : "d86a0604d8680e0fbc001ac756ea7698",
    "deleted" : "false",
    "domain" : "evontech.com",
    "donotIndex" : false,
    "isDomainCard" : false,
    "kiTAG" : "test#evontech.com",
    "lastIndexedOn" : null,
    "manualUpdate" : true,
    "name" : "test",
    "relatedKeywords" : "",
    "seo" : true,
    "status" : "ACTIVE",
    "title" : "test",
    "type" : "Wi-Fi",
    "updatedAt" : ISODate("2015-11-17T06:32:52.320Z")
}
{
    "__v" : 0,
    "_id" : ObjectId("564b21fd5efcf49c204b131f"),
    "additionalProperties" : {
        "urlImage" : "",
        "urlTagDescription" : "%3Cp%3Ethis%20is%20test%3C%2Fp%3E%0A",
        "url" : "http://www.test.com"
    },
    "address" : [ ],
    "businessId" : ObjectId("55c337720545813123acf09f"),
    "clientId" : ObjectId("55c33660cfc3bb56068b4567"),
    "createdAt" : ISODate("2015-11-17T12:47:57.870Z"),
    "creatorAccountId" : ObjectId("55c3366672a63c6a2246af3e"),
    "dataChecksum" : "345674fe5d42fc9e12f6d66ec4cb7f57",
    "deleted" : "564b21fd5efcf49c204b131f",
    "domain" : "evontech.com",
    "donotIndex" : false,
    "isDomainCard" : false,
    "kiTAG" : "testing#evontech.com",
    "lastIndexedOn" : null,
    "manualUpdate" : true,
    "name" : "testing",
    "relatedKeywords" : "",
    "seo" : true,
    "status" : "ACTIVE",
    "title" : "Testing",
    "type" : "URL",
    "updatedAt" : ISODate("2015-11-17T12:47:57.870Z")
}
{
    "additionalProperties" : {
        "urlImage" : "",
        "urlTagDescription" : "%3Cp%3ECheck%20One%3C%2Fp%3E%0A",
        "url" : "http://www.test.com"
    },
    "lastIndexedOn" : null,
    "creatorAccountId" : ObjectId("55c3366672a63c6a2246af3e"),
    "clientId" : ObjectId("55c33660cfc3bb56068b4567"),
    "businessId" : ObjectId("55c337720545813123acf09f"),
    "domain" : "evontech.com",
    "relatedKeywords" : "",
    "name" : "check one",
    "kiTAG" : "checkone#evontech.com",
    "type" : "URL",
    "title" : "Check%20One",
    "_id" : ObjectId("564c42fecf3fcc200ec1806a"),
    "status" : "ACTIVE",
    "isDomainCard" : false,
    "donotIndex" : false,
    "dataChecksum" : "8bc462e65c934f6478bf0f41a6f3e30a",
    "manualUpdate" : true,
    "updatedAt" : ISODate("2015-11-18T09:21:02.296Z"),
    "createdAt" : ISODate("2015-11-18T09:21:02.296Z"),
    "deleted" : "false",
    "address" : [ ],
    "seo" : true,
    "__v" : 0
}
> 

Example-

db.keytags.find( { "kiTAG": { $in: ["checkone#evontech.com","connect#evontech.com",] } } ).pretty()

Output-

{
    "__v" : 0,
    "_id" : ObjectId("56499d341af7854313c2fb8a"),
    "additionalProperties" : {
        "wifiTagDescription" : "",
        "networkType" : "WEP",
        "password" : "Ankit11",
        "ssid" : "ankit.chettri%40evontech.com"
    },
    "businessId" : ObjectId("55c337720545813123acf09f"),
    "clientId" : ObjectId("55c33660cfc3bb56068b4567"),
    "createdAt" : ISODate("2015-11-16T09:09:08.923Z"),
    "creatorAccountId" : ObjectId("55c3366672a63c6a2246af3e"),
    "dataChecksum" : "3d794b6847d0e7b2fcc355462b2e1d95",
    "deleted" : "false",
    "domain" : "evontech.com",
    "donotIndex" : false,
    "isDomainCard" : true,
    "kiTAG" : "connect#evontech.com",
    "lastIndexedOn" : null,
    "manualUpdate" : true,
    "name" : "connect",
    "relatedKeywords" : "",
    "seo" : true,
    "status" : "ACTIVE",
    "title" : "TEST",
    "type" : "Wi-Fi",
    "updatedAt" : ISODate("2014-12-17T06:17:17.461Z")
}
{
    "additionalProperties" : {
        "urlImage" : "",
        "urlTagDescription" : "%3Cp%3ECheck%20One%3C%2Fp%3E%0A",
        "url" : "http://www.test.com"
    },
    "lastIndexedOn" : null,
    "creatorAccountId" : ObjectId("55c3366672a63c6a2246af3e"),
    "clientId" : ObjectId("55c33660cfc3bb56068b4567"),
    "businessId" : ObjectId("55c337720545813123acf09f"),
    "domain" : "evontech.com",
    "relatedKeywords" : "",
    "name" : "check one",
    "kiTAG" : "checkone#evontech.com",
    "type" : "URL",
    "title" : "Check%20One",
    "_id" : ObjectId("564c42fecf3fcc200ec1806a"),
    "status" : "ACTIVE",
    "isDomainCard" : false,
    "donotIndex" : false,
    "dataChecksum" : "8bc462e65c934f6478bf0f41a6f3e30a",
    "manualUpdate" : true,
    "updatedAt" : ISODate("2015-11-18T09:21:02.296Z"),
    "createdAt" : ISODate("2015-11-18T09:21:02.296Z"),
    "deleted" : "false",
    "address" : [ ],
    "seo" : true,
    "__v" : 0
}
> 

However as stated in above the order of the arguments in an array of an $in clause does not fetches the order of how the documents are retrieved as it follows the natural order in which documents are stored as shown in above find clause.