Mongodb aggregate (group) subdocument problem

  mongodb, question

My data:

{
 "_id" : ObjectId("542911f4d7054ab6ee0de476"),
 "name" : "policy2",
 "url" : [ "www.url22.com", "www.url21.com" ],
 "countries" : [
 { "country" : "USA" },
 { "country" : "GER" }
 ]
 }

I would like to count the occurrence of USA and China separately for the group by countries.country field, instead of [USA,GER] appearing together several times, as follows:

{ "_id" : "China", "total" : 1 }
 { "_id" : "USA", "total" : 2 }
 { "_id" : "GER", "total" : 1 }
 { "_id" : "JP", "total" : 1 }

I tried to use aggregate

db.policy.aggregate({$group:{_id:"$countries.country",total:{$sum:1}}})

But the result is the statistical result of countries:

{ "_id" : [ "China" ], "total" : 1 }
 { "_id" : [ "USA", "JP" ], "total" : 1 }
 { "_id" : [ "USA", "GER" ], "total" : 1 }

Seek a solution! !

Use$unwind

db.policy.aggregate([
 { $unwind: '$countries' },
 { $group: { _id: '$countries.country', total: { $sum: 1 } } }
 ]);