db={"users":[{_id:0,name:"abc",phone:999999999},{_id:1,name:"xyz",phone:888888888},],"product":[{_id:"p01",name:"product-name",price:1200},{_id:"p02",name:"product-name1",price:100}],"review":[{_id:"pr0",userId:0,productId:"p01",star:4},{_id:"pr1",userId:1,productId:"p01",star:3}]}

db.product.aggregate([{$lookup:{from:"review",localField:"_id",foreignField:"productId",as:"review",},},{"$unwind":"$review"},{$lookup:{from:"users",localField:"review.userId",foreignField:"_id",as:"review.userInfo",},},])