[{_id:1,name:"NZ",country_info:{phone_system:123,states:[{state_name:"ABC",population:12},{state_name:"DEF",population:22},{state_name:"GHI",population:55}]}},{_id:2,name:"Oz",country_info:{phone_system:456,states:[{state_name:"ABC",population:8},{state_name:"DEF",population:5},{state_name:"GHI",population:3}]}},]

db.collection.aggregate([{"$match":{"country_info.states.state_name":"DEF"}},{"$unwind":"$country_info.states"},{"$match":{"country_info.states.state_name":"DEF"}},{"$project":{"name":1,"population":"$country_info.states.population"}}])