[{"year":"2017","winners":[{"name":"james","id":1},{"name":"adam","id":2}]},{"year":"2018","winners":[{"name":"mary","id":3},{"name":"jane","id":4}]}]

db.collection.aggregate([{$unwind:"$winners"},{$replaceRoot:{newRoot:{$mergeObjects:["$winners",{year:"$year"}]}}}])