Java mongodb aggregation count occurrences

Question

I have some documents like this:

{
  "user": '1'
},
{ "user": '1'
},
{
  "user": '2'
},
{
  "user": '3'
}

I'd like to be able to get a set of all the different users and their respective counts, sorted in decreasing order. So my output would be something like this:

{
  '1': 2,
  '2': 1,
  '3': 1
}

So, in mongodb is :

db.collection.aggregate(
   {$group : { _id : '$user', count : {$sum : 1}}}
).result

But I want to do this in java. I tried this:

AggregateIterable it = collection.aggregate(Arrays.asList(
        group("$id", Accumulators.sum("$user", 1)),
        sort(Sorts.descending("$user"))
));

But it's wrong... How I can do this?

Charchit Kapoor · Accepted Answer · 2021-05-31 16:12:26Z

2

Try this:

AggregateIterable it = collection.aggregate(Arrays.asList(
    group("$user", Accumulators.sum("count", 1)),
    sort(Sorts.descending("count"))
));

answered May 31, 2021 at 16:12

Charchit Kapoor

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2 Comments

Chariot Over a year ago

thanks!, how can I do if I want to show another field of the document?

Charchit Kapoor Over a year ago

You can calculate another field as part of a group like this: group("$customerId", sum("totalQuantity", "$quantity"), avg("averageQuantity", "$quantity")), however, if you want fields from the original document to come up, try using facet.

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