mongodb: find summary of records

I have a collection called 'tickers'

Data is formatted as:

  "_id": ObjectId("50d192b5480cf44157000000"),
  "ticker": {
    "high": 13.38839,
    "low": 13.1225,
    "avg": 13.244705635,
    "vwap": 13.258414151,
    "vol": NumberInt(21562),
    "last_all": 13.57334,
    "last_local": 13.3324,
    "last": 13.3324,
    "buy": 13.33185,
    "sell": 13.3749
  "date": ISODate("2012-12-19T10: 11: 01.439Z")

Data is inserted every minute from the price quotes and transactions.

I want to find the average for every hour from the above data.

I tried:

 'aggregate' => 'tickers',
 'pipeline' => array( 
       '$group' => array( 
        '_id' => array(
            'avg' => array( '$avg' => '$ticker.avg') ,

But only get one record as out put:

   Array ( 
     [result] => Array (
       [0] => Array ( 
             [_id] => Array ( ) 
             [avg] => 13.407843889084 ) ) 
    [ok] => 1 

Expecting at least 100 records for 100 days...


The date operators must be used in a $project operation, not a $group, so you need to do it like this instead (in the shell):

    { $project: {
        _id: 0,
        year: {$year: '$date'},
        month: {$month: '$date'},
        day: {$dayOfMonth: '$date'},
        hour: {$hour: '$date'},
        avg: '$ticker.avg'
    { $group: {
        _id: { year: '$year', month: '$month', day: '$day', hour: '$hour' },
        avg: { $avg: '$avg'}

Gives a result of:

  "result": [
      "_id": {
        "year": 2012,
        "month": 12,
        "day": 19,
        "hour": 10
      "avg": 13.244705635
  "ok": 1

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