CODEWITHSUNDEEP
mongodb
hughesdan
hughesdan

Reputation: 2851

How do you tell Mongo to sort a collection before limiting the results?

I want to select the most recent documents from a collection. The answer from this question mongodb: how to get the last N records? suggests that the order of operations in the query matters. However this does not appear to be the case (perhaps since Mongo 2.4 was released). I've tried both of the following queries and it appears that Mongo first applies the limit and then performs the sort.

Query 1

myCollection.find().sort( { '$date': 1 }).limit(50, callback);

Query 2

myCollection.find().limit(50).sort( { '$date': 1 }, callback);

What would be the correct query to tell mongo to sort first and limit second?

EDIT --- Here's a sample document from the collection

{
  _id: ObjectId("517eb0dddbab79c74700005d"),
  audioFiles: [
    {
      audioFileName: "C64FEFA8-DD43-40A1-8996-35948F3438BF-6896-0000027BD4A59D91",
      audioLanguage: "English",
      date: ISODate("2013-05-21T16:23:04.006Z"),
      flag: "1",
      user: "36C4DEB6-C13D-4211-94B5-CC4DD993ECF1-6896-00000278FA7B08EC"
    },
    {
      audioFileName: "994B6DF6-73B5-458F-912A-FF67A84534B2-23532-0000020000000000",
      audioLanguage: "English",
      date: ISODate("2013-05-27T10:45:04.107Z"),
      flag: "1",
      user: "9D7BB3F4-371B-4F2A-8DA2-0C4CE8B4E16D-974-0000000000000000"
    }
  ],
  date: ISODate("2013-04-29T17:41:49.101Z"),
  imageFileName: "SteamLokomotive0498",
  random: 0.6750695831142366,
  thumbFileName: "SteamLokomotive0498_150x150",
  user: "62923D8E-00CE-4F0C-AECA-3010D78FC9CE-226-0000000000000000",
  userLanguagePref: "Cantonese"
}

Upvotes: 51

Views: 84338

Answers (2)

maudulus
maudulus

Reputation: 11035

I had troubled using the limit, had to remove the $ before it: 

MyCollection.find({ "exampleQuery._id": "exampleId123xx }, { $sort: { requestDate: -1 }, limit: 3 });

Upvotes: 0

JohnnyHK
JohnnyHK

Reputation: 311865

The problem is that you need to sort on date instead of $date.

myCollection.find().sort({date: 1}).limit(50, callback);

Mongo applies the sort before limiting the results regardless of the order you call sort and limit on the cursor.

Proof in docs: link

db.bios.find().sort( { name: 1 } ).limit( 5 )
db.bios.find().limit( 5 ).sort( { name: 1 } )

The two statements are equivalent; i.e. the order in which you chain the limit() and the sort() methods is not significant. Both statements return the first five documents, as determined by the ascending sort order on ‘name’.

Upvotes: 86

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