Можете да направите това с обикновен aggregate
тръбопровод:
MyModel.aggregate([
// Project just the myKey field as that's all that's needed
{$project: {_id: 0, myKey: 1}},
// Duplicate each doc, once per myKey element
{$unwind: '$myKey'},
// Group on myKey and get a count
{$group: {_id: '$myKey', count: {$sum: 1}}}
],
function(err, results) {...}
);