Database
[{"date":"2020-02-24","iso_code":"USA","country":"USA","avg_temperature":"25F",},{"date":"2020-02-25","iso_code":"USA","country":"USA","avg_temperature":"27F",},{"date":"2020-02-24","iso_code":"CHN","country":"China","avg_temperature":"10C",}]
Query
db.collection.aggregate([{"$group":{"_id":"$date","avg_temperatures":{"$mergeObjects":{"$arrayToObject":[[{k:"$country",v:"$avg_temperature"}]]}}}}])