[{_id:1,"name":"Team A","players":[{name:"Michael",position:"Goalkeeper"},{name:"Frank",position:"Defender"},{name:"Mike",position:"Goalkeeper"}]}]

db.collection.aggregate([{$match:{name:"Team A"}},{"$project":{players:{"$reduce":{input:{"$filter":{input:"$players",cond:{$eq:["$$this.position","Goalkeeper"]}}},"initialValue":[],"in":{$concatArrays:["$$value",["$$this.name"]]}}}}}])