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FatalErrorException                                    

Call to a member function getRelationExistenceCountQuery() on null

in QueriesRelationships.php (line 208)    

TopicController.php

<?php

namespace App\Http\Controllers;

use Illuminate\Http\Request;
use App\Topic;

class TopicController extends Controller
{
   public function show(Topic $topic)
   {
       //带文章数的专题
       $topic = Topic::withCount('postTopics')->find($topic->id);

       //专题的文章列表，按照创建时间倒叙排列，前10个
       $posts = $topic->posts()->orderBy('created_at','desc')->take(10)->get();

       return view('topic/show',compact('topic','posts'));
   }

   public function submit(Topic $topic)
   {
       return;
   }
}
Post.php

<?php

namespace App;

use App\Model;

use Laravel\Scout\Searchable;

//表=>posts
class Post extends Model
{
   use Searchable;

   //定义索引里的type
   public function searchableAs()
   {
       return "post";
   }

   //定义有哪些字段需要搜索
   public function toSearchableArray()
   {
       return [
           'title' => $this->title,
           'content' => $this->content,
       ];
   }

   //关联用户
   public function user()
   {
       return $this->belongsTo('App\User');
   }

   //评论模型
   public function comments()
   {
       return $this->hasMany('App\Comment')->orderBy('created_at', 'desc');
   }

   //和用户进行关联
   public function zan($user_id)
   {
       return $this->hasOne(\App\Zan::class)->where('user_id', $user_id);
   }

   //文章的所有赞
   public function zans()
   {
       return $this->hasMany(\App\Zan::class);
   }

   //属于某个作者的文章
   public function scopeAuthorBy(Builder $query, $user_id)
   {
       return $query->where('user_id',$user_id);
   }

   public function postTopics()
   {
       return $this->hasMany(\App\PostTopic::class,'post_id','id');
   }
   //不属于某个专题的文章
   public function scopeTopicNotBy(Builder $query,$topic_id)
   {
       return $query->doesntHave('postTopics','and',function($q) use($topic_id){
           $q->where('topic_id',$topic_id);
       });
   }
}

Topic.php

<?php

namespace App;

use App\Model;

use Laravel\Scout\Searchable;

//表=>posts
class Post extends Model
{
   use Searchable;

   //定义索引里的type
   public function searchableAs()
   {
       return "post";
   }

   //定义有哪些字段需要搜索
   public function toSearchableArray()
   {
       return [
           'title' => $this->title,
           'content' => $this->content,
       ];
   }

   //关联用户
   public function user()
   {
       return $this->belongsTo('App\User');
   }

   //评论模型
   public function comments()
   {
       return $this->hasMany('App\Comment')->orderBy('created_at', 'desc');
   }

   //和用户进行关联
   public function zan($user_id)
   {
       return $this->hasOne(\App\Zan::class)->where('user_id', $user_id);
   }

   //文章的所有赞
   public function zans()
   {
       return $this->hasMany(\App\Zan::class);
   }

   //属于某个作者的文章
   public function scopeAuthorBy(Builder $query, $user_id)
   {
       return $query->where('user_id',$user_id);
   }

   public function postTopics()
   {
       return $this->hasMany(\App\PostTopic::class,'post_id','id');
   }
   //不属于某个专题的文章
   public function scopeTopicNotBy(Builder $query,$topic_id)
   {
       return $query->doesntHave('postTopics','and',function($q) use($topic_id){
           $q->where('topic_id',$topic_id);
       });
   }
}