Hi all. I need help with $project pipeline operator. I have some document
like this:

{
'_id': '1231231',
'campaign':{
    'id': '321321',
    'title': 'some title'
}
}

I need group by 'campaign.id' and in result i need 'campaign.title' field.
My code:


$opts = array(
    array('$match' => $condition),
    array('$project' => array(
        'campaign' => 1,
    )),
    array('$group' => array(
        '_id' => '$campaign.id',
       'title' => array('$first' => '$campaign.title'),
   ))
);

It's worked, but i am not sure in this line " 'title' => array('$first' =>
'$campaign.title'),"

When i tried " 'title' => '$campaign.title'," in $group operator i got
error "the group aggregate field 'title' must be defined as an expression
inside an object"

This code not worked to:

    array('$project' => array(
        'campaign' => 1,
        'title' => '$campaign.title'
    )),

No errors. But in result i cant see "title" field.

What is wrong? Thanks and sorry for my bad English.

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groupmongodb-user @
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postedSep 6, '13 at 7:52a
activeSep 6, '13 at 10:48a
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