mongodb在查找查询中将_id作为字符串获取

Question

Here I have created a collection with a single document

db.getCollection('example').insert({"example":1});

I have tried to use Projection, and I get back the _id.

db.getCollection('example').find({"example":1},{"_id":1});

{
    "_id" : ObjectId("562a6300bbc948a4315f3abc")
}

However, I need the below output as shown below.

1. id and not _id

2. ObjectId("562a6300bbc948a4315f3abc") vs "562a6300bbc948a4315f3abc"

   { "id" : "562a6300bbc948a4315f3abc" }

Although I can process #1 and #2 on my app server(PHP based) to get the desired ouput, I am looking if there is a way to get the expected result on querying from mongo itself

Answer 1

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.

Answer 2

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
        )

)

Answer 3

One simple solution for traversing MongoCursor on PHP side is to use Generators as well as foreach or array_map($function, iterator_to_array($cursor)) . Example:

function map_traversable(callable $mapper, \Traversable $iterator) {
    foreach($iterator as $val) {
        yield $mapper($val);
    }
}

You can meet more at PHP documentation about generators syntax .

So, now you can use/reuse it (or similar implementation) for any propose of "projecting" your data on PHP side with any amount of mapping (just like pipeline does in aggregate ) but with fewer iterations amount. And this solution is pretty convenient for OOP in a case of reusing your map functions.

UPD: Just for your case example below:

$cursor = $db->getCollection('example')->find(["example":1],["_id":1]);
$mapper = function($record) {
    return array('id' => (string) $record['_id']); //see \MongoId::__toString()
}
$traversableWithIdAsStringApplied = map_traversable($mapper, $cursor);
//...

now you can proceed with more mappings applied to $traversableWithIdAsStringApplied or use just iterator_to_array for simple array retrieving.