lachang/database/seeds/UpdateCommentNickName.php

<?php

use app\common\model\Comment;
use think\facade\Db;
use think\migration\Seeder;

class UpdateCommentNickName extends Seeder
{
    /**
     * Run Method.
     *
     * Write your database seeder using this method.
     *
     * More information on writing seeders is available here:
     * http://docs.phinx.org/en/latest/seeding.html
     */
    public function run()
    {
        $comments = Comment::with(['user'])->select();
        foreach($comments as $item) {
            $item->nick_name = $item->user->nick_name ?? '';
            $item->save();
        }
    }
}