MongoDB projection parameter not working in findOne()

Question

I'm trying to use a projection parameter on findOne() to extract a single field from a document (stats) but it just seems to return the whole document. I'm using version "mongodb": "^3.4.1" in Node.js

This is the document structure

{ _id: 5e563015fa9a1a0134cac3cb,
  username: 'user1',
  password: '1234',
  email: '[email protected]',
  stats: 
   { totalViewed: 122,
     totalUnique: 4,
     tknow: 80,
     tdknow: 42,
     setCnt: 78 },
  progress: 
   [ { cardId: 1001, knowCnt: 3, dknowCnt: 4 },
     { cardId: 1016, knowCnt: 0, dknowCnt: 0 } ] }

This is the code:

 var findOneDoc = function() {
        db.collection("testusers").findOne(
          { username: "user1" },
          { stats: 1 }, //field to return
          function(err, result) {
            if (err) {
              console.log("Error: ", err);
            }
            console.log("Success: ", result);
          }
        );
      };
     findOneDoc();

I also tried:{$project: {stats: 1}}, to no avail

Thanks

As a double check, did you try db.collection["testusers'].findOne({"username":"user1"},{stats:1}) directly from the shell? — Buzz Moschetti
– Buzz Moschetti, Commented Mar 5, 2020 at 2:10
Sorry I'm not familiar with using the shell directly. Thanks — quietplace
– quietplace, Commented Mar 5, 2020 at 3:14

mickl · Accepted Answer · 2020-03-05 01:48:40Z

56

Based on the documentation the .findOne() method takes options as a second parameter and it is recommended to use projection to define fields:

db.collection("testusers").findOne(
    { username: "user1" },
    { projection: { stats: 1 } },
    function(err, result) { ... }
);

answered Mar 5, 2020 at 1:48

mickl

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6 Comments

quietplace Over a year ago

Thanks. When I tried this nothing returned from the database - so success or error messages or fields returned.

var findOneDoc4 = function() {       db.collection("testusers").findOne(     { username: "user1" },     { projection: { stats: 1 } },     function(err, result) {          if (err) {               console.log("Error: ", err);             }             console.log("Success: ", result);      }   );

quietplace Over a year ago

The document is shown at the top in the original question. Thanks

mickl Over a year ago

what if you remove second parameter, are you still getting no result?

quietplace Over a year ago

If I remove the second parameter the whole document returns as expected.

mickl Over a year ago

Can you try { fields: { stats: 1 } } instead of { projection: { stats: 1 } }?

|

Stanley · Accepted Answer · 2021-08-12 07:47:42Z

4

with findOne operations, the second parameter passed is an options parameter, so you should pass in your projects within the options parameter, like this:

query = { username: "user1" };
options = { projection: { stats: 1 }};

db.collection("testusers").findOne(query, options)

edited Aug 12, 2021 at 7:47

Stanley

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answered Aug 11, 2021 at 16:20

Bryant

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1 Comment

Iglesias Leonardo Over a year ago

Don't forget to await

zvijude · Accepted Answer · 2022-05-16 18:03:33Z

3

you can do it like mick said, with findOne

await db.collection("testusers").findOne(
    { username: "user1" },
    { projection: { stats: 1 } },
    function(err, result) { ... }
);

or you can do it with find, like that

await db.collection("testusers")
.find({ username: "user1" })
.limit(1)
.project(["stats"])
.toArray()

you can add fields to the array project for more fields

edited May 16, 2022 at 18:03

answered May 16, 2022 at 17:56

zvijude

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Comments

Cahaba Data · Accepted Answer · 2020-03-05 03:18:54Z

-1

findOne returns a document. use find() …..

answered Mar 5, 2020 at 3:18

Cahaba Data

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1 Comment

quietplace Over a year ago

I tried .find() but it still didn't return anything. I'm only targeting one document so my understanding is that findOne() should work and is compatible with projection parameters. Thanks

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