How to get a specific object from array of objects inside specific MongoDB document?

Question

There is struct of my MongoDB documents:

{
    _id : "12345",
    name : "foo",
    object : {
        array_of_objects : [{
            id : 507,
            name : "some name",
            dt : "2012-06-27 16:35:50"
        },{
            id : 506
            name : "some other name",
            dt : "2012-06-21 16:09:05"
        },
        …
        ]
    }
}

I need to get an object from array_of_objects with a certain id of the document with the specified name. I use php and tried to execute the next code:

$collection->find(array('name' => 'foo', 'object.array_of_objects.id' => 507));

It returns all elements of array_of_objects instead of element with id 507. After that I try to make query with $elemMatch:

$collection->find(array('name' => 'foo', 'object.array_of_objects' => array('$elemMatch' => array('id' => 507))));

But it had return the same. :( My MongoDB version 2.0.6. Please help.

Drawback of MongoDB: stackoverflow.com/questions/10042097/… — David Cheung
– David Cheung, Commented Jun 28, 2012 at 18:10
@david-cheung thanks! So, I'll try to do this with Mapreduce. — bibimij
– bibimij, Commented Jun 28, 2012 at 19:15

Jenna · Accepted Answer · 2012-06-28 19:58:57Z

3

This issue has been resolved and will be available in MongoDB version 2.2, the next stable release: https://jira.mongodb.org/browse/SERVER-828

I just tried it using MongoDB 2.1.2 (unstable, development release):

sample doc:

{ "_id" : 1, "object" : { "array" : [ { "id" : 507, "name" : "Jenna" }, { "id" : 506, "name" : "Matt" } ] } }

query:

db.food.find({_id:1, "object.array.id":506},{_id:0, "object.array.$":1})

result:

{ "object" : { "array" : [ { "id" : 506, "name" : "Matt" } ] } }

edited Jun 28, 2012 at 19:58

answered Jun 28, 2012 at 19:27

Jenna

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1 Comment

bibimij Over a year ago

Good news everyone! 8) Thanks, @Jenna!

Blacksad · Accepted Answer · 2012-06-28 18:47:39Z

0

You cannot do this with MongoDB: MongoDB won't drill down into a particular document to extract an item from an array for instance, you have to do this by yourself.

Now the PHP driver might do more work, I don't know -- I'm speaking about MongoDB core queries.

edited Jun 28, 2012 at 18:47

answered Jun 28, 2012 at 18:03

Blacksad

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3 Comments

Jenna Over a year ago

You can return a subset of fields- check out the MongoDB documentation: mongodb.org/display/DOCS/Retrieving+a+Subset+of+Fields

Blacksad Over a year ago

@Jenna yes you're right -- but a specific item from an array based on its value ?

bibimij Over a year ago

As I understand, it can be done either by getting the entire array of objects and the search for the desired item using PHP, or by using MapReduce, right?

bibimij · Accepted Answer · 2012-06-29 12:38:48Z

0

So, I did it with MapReduce. Hopefully, this will save someone time and nerves.

$name = 'foo';
$id = 507;

$mongo = new Mongo('…');

$db = $mongo->my_db;

$map = new MongoCode('function(){'.
    'this.object.array_of_objects.forEach(function(obj){'.
        'if (obj.id == '.$id.')'.
            'emit(1, obj);'.
    '});}'
);

$reduce = new MongoCode('function(k,v){ return [k,v]; }');

$response = $db->command(array(
    'mapreduce' => 'my_collection', 
    'map' => $map,
    'reduce' => $reduce,
    'query' => array('name' => $name),
    'out' => array('inline' => 1)
));

var_dump($response['result']);

edited Jun 29, 2012 at 12:38

answered Jun 29, 2012 at 1:01

bibimij

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