php - mongo sort after limit after sort - Not working

Question

I have a collection, from which i get particular type of users using $query
Then I need sort them according to user_id ascending and limit them to 2000

From these I need the max user_id, so I sort them in descending order and limit to 1.

But this second sort forgets the limit of 2000 and sorts over over the entire cursor from find().

Any work-around?

$cursor = $collection   ->find($query)                // too many entries
            ->sort(array('user_id'=>1))   // go ascending
            ->limit(2000)                // got our limited qouta
            ->sort(array('user_id'=>-1)) // go descending to get max
            ->limit(1);          // the one with max(user_id)

Answer 1

Your cannot do a sort and then a limit and then a sort. The Cursor object is exactly that and it will not run the query until you iterate to the first result via getNext() which is either run manually or within a foreach loop not only that but sort is just a property of the object as such making two sorts just overwrites the property.

The best way to achieve what your looking for is:

$doc = $collection->find($query)->sort(array('user_id' => 1))
       ->skip(1999)->limit(1)->getNext();

That will always pick the highest user_id (which occurs at the end in this sort) of the group, which will give off the same results as doing two sorts.