weappadmin-api/app/Models/Message.php

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class Message extends Model
{
    protected $table='message';
    protected $fillable=['user_id','activity_id','content','pid'];

    public function user(){
        return $this->belongsTo(User::class,'user_id');
    }

    public function messages()
    {
        return $this->hasMany(Message::class, 'pid');
    }
}