Question

I have a MongoDB collection with number of items. Multiple items have a same key vlue(srkey), but value of their timestamp is different. I want to to fetch all items by srkey with greatest timestamp. How to accomplish this with php MongoDB.

For example the my collection contains following items

• collection

  item1 : {srkey:a,timestamp:1358675171}
  item2 : {srkey:b,timestamp:1358675175}
  item3 : {srkey:a,timestamp:1358676849}
  item4 : {srkey:d,timestamp:1358675213}
  item5 : {srkey:b,timestamp:1358676111}
  

Output of fetch must be

    item3 : {srkey:a,timestamp:1358676849}
    item4 : {srkey:d,timestamp:1358675213}
    item5 : {srkey:b,timestamp:1358676111}

Item 1 and 3 have same srkey, so the result contains only one of the them with greatest timestamp value.

Answer 1

You can use the new aggregation framework for this:

$db->col->aggregate(array(
    array('$sort' => array('$timestamp' => -1)),
    array('$group' => array('_id' => '$srkey', 
        'timestamp' => array('$first' => '$timestamp')))
));

It should be the first of that sorted group since you are sorting in a DESC manner.