How to aggregate on huge array in mongoDB?

Question

I have a mongodb of about 400gb. The documents contain a variety of fields, but the key here is an array of IDs.

So a json file might look like this

Answer 1

Try this:

db.users.aggregate( 
 [ 
  { $unwind : "$key" }, 
  { $group : { _id : "$key", number : { $sum : 1 } } },
  { $sort : { number : -1 } }, 
  { $limit : 10000 },
  { $out:"result"},
 ], {
  allowDiskUse:true,
  cursor:{}
 }
);

Then find result by db.result.find().