db={"employess":[{_id:1,name:"John Doe",filesAccess:{web:true},fileIds:["fileId1","fileId2"]},{_id:2,name:"Bob Jones",filesAccess:{web:false},fileIds:["fileId3","fileId4"]}],"files":[{_id:"fileId1",fileMetaData:{location:"NE"}},{_id:"fileId2",fileMetaData:{location:"NE"}},{_id:"fileId3",fileMetaData:{location:"SW"}},{_id:"fileId4",fileMetaData:{location:"SW"}}]}

db.employess.aggregate([{"$match":{"filesAccess.web":true}},{"$lookup":{"from":"files","localField":"fileIds","foreignField":"_id","as":"result"}},{"$set":{"result":{"$arrayElemAt":["$result",0]}}},{"$project":{"_id":1,"name":1,"location":"$result.fileMetaData.location"}}])