laravel怎么实现某个字段排名

php artisan make:migration add_rank_field_to_students_table --table=students

public function up(){    Schema::table('students', function (Blueprint $table) {        $table->unsignedInteger('rank')->nullable();    });}public function down(){    Schema::table('students', function (Blueprint $table) {        $table->dropColumn('rank');    });}

public function scopeRank($query, $field){    $query->orderByDesc($field);    $query->select($field);    $query->addSelect(DB::raw('(@rank := @rank + 1) as rank'));    $query->from(DB::raw('(' . $query->toSql() . ') as res'));    $query->join(DB::raw('(select @rank := 0) as init'), function () {    });}

$students = Student::rank('score')->get();foreach ($students as $student) {    $student->rank = $student->pivot->rank;    $student->save();}