[{"_id":"614443e922b33330542a2645","userName":"John","code":1,},{"_id":"614443e922b33330542a2646","userName":"Adam","code":2,},{"_id":"614443e922b33330542a2647","userName":"Lily","code":3,},{"_id":"614443e922b33330542a2649","userName":"Brad","code":5,}]

db.collection.aggregate([{$group:{_id:"Test",arr:{$push:"$code"},ma:{$max:"$code"},mi:{$min:"$code"}}},{$project:{_id:0,code:{$min:{"$setDifference":[{$range:["$mi","$ma",1]},"$arr"]}}}}])