[{"fruits":{"brasil":["pineapple","mango",],"france":["apple","grape"]}}]

db.collection.aggregate([{"$set":{"fruits":{"$objectToArray":"$fruits"}}},{$set:{fruits:{"$reduce":{"input":"$fruits","initialValue":[],"in":{"$concatArrays":["$$value",{"$map":{"input":"$$this.v","as":"f","in":{$concat:["$$this.k","_","$$f"]}}}]}}}}}])