mongo/jstests/aggregation/optimization/sort_limit_indexed.js

// server-7900 - $sort + $limit ignores limit when using index for sort

let c = db[jsTestName()];
c.drop();

for (var i = 0; i < 5; i++)
    c.insert({_id: i});

let res = c.aggregate({$sort: {_id: -1}}, {$limit: 2});  // uses index for sort
assert.eq(res.toArray(), [{_id: 4}, {_id: 3}]);