mongo排序后限制后排序-不起作用 [英] mongo sort after limit after sort - Not working

问题描述

我有一个集合，使用$ query从中可以得到特定类型的用户
然后我需要根据user_id升序对它们进行排序并将其限制为2000

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

从这些中我需要最大的user_id，因此我将它们降序排列并限制为1.

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

但是第二种排序忘记了2000的限制，并通过find()在整个游标上进行排序.

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

有其他解决方法吗?

$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)

推荐答案

您不能先进行排序，再进行限制，然后进行排序. Cursor对象正是该对象，直到您通过getNext()迭代到第一个结果(它是手动运行或在foreach循环内运行)时，它才会运行查询，不仅如此，而且sort仅仅是对象的一个​​属性因此，进行两种排序只会覆盖该属性.

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.

实现您所寻找的最佳方法是:

The best way to achieve what your looking for is:

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

这将始终选择组中最高的user_id(在此类别的末尾出现)，其结果将与执行两种排序相同.

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.

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