laravel管理模型插入

post控制器
public function comment(Post $post,Request $request)
{
   try{
      if(empty($request->content)){
         ExceptionResult::throwException(ErrorCode::COMMENT_EMPTY);
      }

      $comment = new Comment();
      $comment->content = $request->content;
      $comment->user_id = Auth::id();
      $res = $post->comments()->save($comment);
//还有种
$comment->post_id = $post->id;
$res = $comment->save();
      if($res){
         return  Redirect::back()->with('success', '评论成功');
      } else {
         ExceptionResult::throwException(ErrorCode::ACTION_FAIL);
      }

   }catch (ExceptionResult $e){

      return  Redirect::back()->with('error', $e->getMessage())->withInput();
   }
}

post模型

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class Post extends Model
{
    //

   protected $table = "posts";
   public $primaryKey = 'id';

   public function user()
   {
      return $this->belongsTo("App\Models\User","user_id",'id');
   }

   public function comments()
   {
      return $this->hasMany('App\Models\Comment','post_id','id')->orderBy("created_at",'desc');
   }
}