[{"_id":1.0,"date":"2020-05-10","book":[{"price":NumberDecimal("10.0"),"ISBN":"a1","qty":10},{"price":NumberDecimal("100.0"),"ISBN":"a2","qty":10}]}]

db.collection.aggregate([{$addFields:{book:{$map:{input:"$book",in:{$mergeObjects:["$$this",{total:{$multiply:["$$this.qty","$$this.price"]}}]}}}}}])