Database
db={"journals":[{title:"String",category:"String",subcategory:"String",review:[ObjectId("5a934e000102030405000001"),ObjectId("5a934e000102030405000002")],link:"String",description:"String",subscribers:"Number",image:"String"}],"reviews":[{"_id":ObjectId("5a934e000102030405000001"),author:"",content:"String",date:"Date",rating:4},{"_id":ObjectId("5a934e000102030405000002"),author:"",content:"String",date:"Date",rating:5}]}
Query
db.journals.aggregate([{$lookup:{from:"reviews",localField:"review",foreignField:"_id",as:"review"}},{$addFields:{review:{$avg:"$review.rating"}}},{$match:{review:{$gt:4}}}])