Database

db={"reviews":[{followers:[],reviews:[{"author":"5e9167c5303a530023bcae42","rate":5,"spoiler":false,"content":"This is a comment This is a comment This is a comment.","createdAt":"2020-04-12T16:08:34.966Z","updatedAt":"2020-04-12T16:08:34.966Z"},{"author":"5e9167c5303a530023bcae46","rate":4,"spoiler":false,"content":"This is a comment This is a comment This is a comment1.","createdAt":"2020-04-12T16:08:34.966Z","updatedAt":"2020-04-12T16:08:34.966Z"},{"author":"5e9167c5303a530023bcae45","rate":3,"spoiler":false,"content":"This is a comment This is a comment This is a comment2.","createdAt":"2020-04-12T16:08:34.966Z","updatedAt":"2020-04-12T16:08:34.966Z"}]}],"users":[{"_id":"5e9167c5303a530023bcae42",name:"abc"},{"_id":"5e9167c5303a530023bcae45",name:"def"}]}

Query

db.reviews.aggregate([{$unwind:{path:"$reviews",preserveNullAndEmptyArrays:true}},{$lookup:{from:"users",localField:"reviews.author",foreignField:"_id",as:"author"}},{$addFields:{"reviews.author":{$cond:[{$ne:["$author",[]]},{$arrayElemAt:["$author",0]},"$reviews.author"]}}},{$group:{_id:"$_id",data:{$first:"$$ROOT"},reviews:{$push:"$reviews"}}},{$addFields:{"data.reviews":"$reviews"}},{$project:{"data.author":0}},{$replaceRoot:{newRoot:"$data"}}])

Result