[{_id:ObjectId("5e388da4df54cb8efb47e61b"),userId:"test_user",productId:"product_6_id",rankedList:[{id:"product_5_id",Name:"Product_5"},{id:"product_6_id",Name:"Product_6"},{id:"product_3_id",Name:"Product_3"}]}]

db.collection.aggregate([{$project:{index:{$indexOfArray:["$rankedList.id","product_3_id"]}}}])