[{"iso2":"IN","states":[{id:1,name:"Region",state_code:"AG",cities:[{id:11,name:"Agrokavan"},{id:22,name:"Brussels"},{id:33,name:"Sofia"},{id:44,name:"Tokyo"}]},{id:1,name:"Region2",state_code:"AB",cities:[{id:55,name:"Mexico"},{id:66,name:"Madrid"},{id:77,name:"Roma"},{id:88,name:"Barcelona"}]}]}]

db.collection.aggregate([{"$match":{"iso2":"IN","states.state_code":"AG"}},{$addFields:{states:{"$filter":{"input":"$states","as":"state","cond":{"$eq":["$$state.state_code","AG"]}}}}},{$project:{cities:"$states.cities.name"}},{$unwind:"$cities"}])