db={"Product":[{id:1,URL:"https://stackoverflow.com/questions/ask"},{id:2,URL:"https://stackoverflow.com/questions/ask100?=view"},{id:3,URL:"https://stackoverflow.com/questions/ask101"}]}

db.Product.aggregate([{$project:{id:1,URL:{$arrayElemAt:[{"$split":["$URL","?"]},0]}}}])