db.beer.aggregate( [
{ '$group' : {
'_id' : '$brewery_country',
'beers' : { '$addToSet' : '$beer_name' },
} },
{ '$unwind' : '$beers' },
{ '$group' : { '_id' : '$_id', 'count' : { '$sum' : 1 } } },
] ).pretty()
{ "_id" : "Denmark", "count" : 31 }
{ "_id" : "Czech Republic", "count" : 19 }
{ "_id" : "Spain", "count" : 20 }
{ "_id" : "Argentina", "count" : 13 }
{ "_id" : "Dominican Republic", "count" : 1 }
{ "_id" : "Mexico", "count" : 7 }
{ "_id" : "Norway", "count" : 60 }
{ "_id" : "Greece", "count" : 5 }
{ "_id" : "New Zealand", "count" : 6 }
{ "_id" : "India", "count" : 1 }
{ "_id" : "Australia", "count" : 5 }
{ "_id" : "Netherlands", "count" : 79 }
{ "_id" : "Italy", "count" : 20 }
…
