db={"UserReview":[{_id:1,username:"John",content:"I liked the product"}],"UserReply":[{_id:11,username:"Andy",content:"I liked the product",userReviewId:1,status:"REJECTED"},{_id:12,username:"Bob",content:"cool",userReviewId:1,status:"APPROVED"}]}

db.UserReview.aggregate([{"$lookup":{"from":"UserReply","let":{id:"$_id"},"pipeline":[{$match:{$expr:{$eq:["$$id","$userReviewId"]},status:"APPROVED"}}],"as":"replies"}}])