[{"_id":1,"recommended":[{"movieId":"1","recommendedBy":"X"},{"movieId":"2","recommendedBy":"Y"},{"movieId":"3","recommendedBy":"Z"}]}]

db.collection.update({},[{$set:{recommended:{"$reduce":{"input":"$recommended","initialValue":/** assign your values here:*/[{"movieId":"1","recommendedBy":"The id"}],"in":{"$cond":{"if":{$in:["$$this.movieId","$$value.movieId"]},"then":"$$value","else":{"$concatArrays":["$$value",["$$this"]]}}}}}}}])