[mongodb] MongoDB SELECT COUNT GROUP BY

I am playing around with MongoDB trying to figure out how to do a simple

SELECT province, COUNT(*) FROM contest GROUP BY province

But I can't seem to figure it out using the aggregate function. I can do it using some really weird group syntax

db.user.group({
    "key": {
        "province": true
    },
    "initial": {
        "count": 0
    },
    "reduce": function(obj, prev) {
        if (true != null) if (true instanceof Array) prev.count += true.length;
        else prev.count++;
    }
});

But is there an easier/faster way using the aggregate function?

Additionally if you need to restrict the grouping you can use:

db.events.aggregate( 
    {$match: {province: "ON"}},
    {$group: {_id: "$date", number: {$sum: 1}}}  
)

I need some extra operation based on the result of aggregate function. Finally I've found some solution for aggregate function and the operation based on the result in MongoDB. I've a collection Request with field request, source, status, requestDate.

Single Field Group By & Count:

db.Request.aggregate([
    {"$group" : {_id:"$source", count:{$sum:1}}}
])

Multiple Fields Group By & Count:

db.Request.aggregate([
    {"$group" : {_id:{source:"$source",status:"$status"}, count:{$sum:1}}}
])

Multiple Fields Group By & Count with Sort using Field:

db.Request.aggregate([
    {"$group" : {_id:{source:"$source",status:"$status"}, count:{$sum:1}}},
    {$sort:{"_id.source":1}}
])

Multiple Fields Group By & Count with Sort using Count:

db.Request.aggregate([
    {"$group" : {_id:{source:"$source",status:"$status"}, count:{$sum:1}}},
    {$sort:{"count":-1}}
])

Mongo shell command that worked for me:

db.getCollection(<collection_name>).aggregate([{"$match": {'<key>': '<value to match>'}}, {"$group": {'_id': {'<group_by_attribute>': "$group_by_attribute"}}}])

This type of query worked for me:

 db.events.aggregate({$group: {_id : "$date", number:  { $sum : 1} }} )

See http://docs.mongodb.org/manual/tutorial/aggregation-with-user-preference-data/

Starting in MongoDB 3.4, you can use the $sortByCount aggregation.

Groups incoming documents based on the value of a specified expression, then computes the count of documents in each distinct group.

https://docs.mongodb.com/manual/reference/operator/aggregation/sortByCount/

For example:

db.contest.aggregate([
    { $sortByCount: "$province" }
]);

If you need multiple columns to group by, follow this model. Here I am conducting a count by status and type:

  db.BusinessProcess.aggregate({
    "$group": {
        _id: {
            status: "$status",
            type: "$type"
        },
        count: {
            $sum: 1
        }
    }
   })