Where did you taste the most *distinct* beers?
*group by* country and *collect* beer names
*create documents* out of collection
*group by* country and *count* items
Pipeline:
db.beer.aggregate( [
{ '$group' : {
'_id' : '$brewery_country',
'beers' : *{ '$addToSet' : '$beer_name' }*,
} },
{ *'$unwind' : '$beers'* },
{ '$group' : { '_id' : *'$_id'*, 'count' : { '$sum' : 1 } } },
{ '$sort' : { 'count' : -1 } },
] ).pretty()