[{"id":"123","favoriteteams":["Yankees","Knicks"],"home":"NY"}]

db.collection.aggregate([{"$project":{favoriteteams:1}},{"$addFields":{obj2:"$$ROOT",obj1:{"name":"max","age":26,"hometown":"NY"}}},{"$project":{merged:{"$mergeObjects":["$obj1","$obj2"]}}},{"$replaceRoot":{"newRoot":"$merged"}}])