mongodb get _id as string in find query

MongoDB 4.0 adds the $convert aggregation operator and the $toString alias which allows you to do exactly that:

db.getCollection('example').aggregate([
  { "$match": { "example":1 } },
  { "$project": { "_id": { "$toString": "$_id" } } }
])

A main usage would most likely be though to use the _id value as a "key" in a document.

db.getCollection('example').insertOne({ "a": 1, "b": 2 })

db.getCollection('example').aggregate([
  { "$replaceRoot": {
    "newRoot": {
      "$arrayToObject": [
        [{ 
          "k": { "$toString": "$_id" },
          "v": {
            "$arrayToObject": {
              "$filter": {
                "input": { "$objectToArray": "$$ROOT" },
                "cond": { "$ne": ["$$this.k", "_id"] }
              }
            }
          }
        }] 
      ]
    }
  }}
])

Which would return:

{ 
  "5b06973e7f859c325db150fd" : { "a" : 1, "b" : 2 }
}

Which clearly shows the string, as does the other example.

Generally though there is usually a way to do "transforms" on the cursor as documents are returned from the server. This is usually a good thing since an ObjectId is a 12-byte binary representation as opposed to a 24 character hex "string" which takes a lot more space.

The shell has a .map() method

db.getCollection('example').find().map(d => Object.assign(d, { _id: d._id.valueOf() }) )

And NodeJS has a Cursor.map() which can do much the same thing:

let cursor = db.collection('example').find()
    .map(( _id, ...d }) => ({ _id: _id.toString(), ...d }));

while ( await cursor.hasNext() ) {
  let doc = cursor.next();
  // do something
})

And the same method exists in other drivers as well ( just not PHP ), or you can just iterate the cursor and transform the content as is more likely the best thing to do.

---

In fact, whole cursor results can be reduced into a single object with great ease by simply adding to any cursor returning statement, when working in the shell

.toArray().reduce((o,e) => { 
  var _id = e._id;
  delete e._id;
  return Object.assign(o, { [_id]: e })
},{ })

Or for full ES6 JavaScript supporting environments like nodejs:

.toArray().reduce((o,({ _id, ...e })) =>  ({ ...o, [_id]: e }),{ })

Really simple stuff without the complexity of what needs to process in the aggregation framework. And very possible in any language by much the same means.

You need to use the .aggregate() method.

db.getCollection('example').aggregate([ { "$project": { "_id": 0, "id": "$_id" } } ]);

Which yields:

{ "id" : ObjectId("562a67745488a8d831ce2e35") }

or using the .str property.

db.getCollection('example').find({"example":1},{"_id":1}).map(function(doc) { 
    return {'id': doc._id.str }
})

Which returns:

[ { "id" : "562a67745488a8d831ce2e35" } ]

Well if you are using the PHP driver you can do something like this:

$connection = new MongoClient();
$db = $connection->test;
$col = $db->example;
$cursor = $col->find([], ["_id" => 1]);
foreach($cursor as $doc) { print_r(array("id" => $doc["_id"])); } 

Which yields:

Array
(
    [id] => MongoId Object
        (
            [$id] => 562a6c60f850734c0c8b4567
        )

)

Or using again the MongoCollection::aggregate method.

$result = $col->aggregate(array(["$project" => ["id" => "$_id", "_id" => 0]]))

Then using the foreach loop:

Array
(
    [_id] => MongoId Object
        (
            [$id] => 562a6c60f850734c0c8b4567
        )

)