Database
[{"_id":"1","Continent":"Asia","Countries":{"South Korea":{"GDP":80,"HGI":60,"FX":10},"India":{"GDP":75,"HGI":112,"FX":28},"Singapore":{"GDP":34,"HGI":60,"FX":0}}}]
Query
db.collection.aggregate([{"$project":{"countries":{"$objectToArray":"$Countries"}}},{"$project":{"countries.v.GDP":1}}])