请教大家，怎么在验证器中指定数据库中的数据表，去验证用户名唯一性？

项目里边有两个数据库，现在是在指定的数据库mysql_wx上重构，
下边代码里边验证会出错，有这个提示
Illuminate\Database\QueryException

SQLSTATE[HY000] [1045] Access denied for user ‘forge‘@’localhost’ (using password: NO) (SQL: select count(*) as aggregate from user where username = admin)

请怎么怎么在这个代码
Rule::unique(‘yt_user’, ‘username’) //用户名不能重复
里边指定数据库mysql_wx的数据表，谢谢！

<?php

namespace App\Http\Requests;

use Illuminate\Validation\Rule;
use Illuminate\Foundation\Http\FormRequest;

class YtuserRegister extends FormRequest
{
    /**
     * Determine if the user is authorized to make this request.
     *
     * @return bool
     */
    public function authorize()
    {
        return true;
    }

    /**
     * Get the validation rules that apply to the request.
     *
     * @return array
     */
    public function rules()
    {
        $rules = [
            'username'  => [
                'required',
                Rule::unique('user', 'username')  //用户名不能重复
            ],
            'password'  => 'required|same:password2',   //密码不能为空，要相同
            'nickname'  => 'required',   //昵称不能为空
        ];

        return $rules;
    }

    //框架不知道的字段名增加上去 对应的中文
    public function attributes()
    {
        return [
            'password2' => '确认密码',
            'nickname' => '昵称',
        ];
    }
}