Database
[{"_id":{"$oid":"6146ada531696ee91a3f9fa4"},"StateName":"Colorado","StateCode":"CO","Cities":[{"_id":{"$oid":"6146ada531696ee91a3f5a50"},"CityName":"Denver","Latitude":"39.55666000","Longitude":"-104.89609000"}]}]
Query
db.collection.aggregate([{"$match":{"Cities.CityName":"Denver"}},{"$addFields":{"Cities":{"$map":{"input":{"$filter":{"input":"$Cities","cond":{"$eq":["$$this.CityName","Denver"]}}},"in":"$$this._id"}}}}])