Laravel, Call to a member function sync() on null - laravel

Question

I have a FatalThrowableError in my laravel projects - Call to a member function sync() on null. Debuger show mi, that is this line of code
Post::findOrFail($post_id)->tags()->sync([1, 2, 3], false);
Complete method this line look like:
public function store(Request $request)
{
// validate a post data
$this->validate($request, [
'title' => 'required|min:3|max:240',
'body' => 'required|min:50',
'category' => 'required|integer',
]);
// store post in the database
$post_id = Post::Create([
'title' => $request->title,
'body' => $request->body,
])->id;
Post::findOrFail($post_id)->tags()->sync([1, 2, 3], false);
// rediect to posts.show pages
return Redirect::route('posts.show', $post_id);
}
My Post model its looks like
class Post extends Model
{
protected $fillable = [ ... ];
public function category() {...}
public function tags()
{
$this->belongsToMany('App\Tag', 'post_tag', 'post_id', 'tag_id');
}
}
And my Tag model its look like
class Tag extends Model
{
protected $fillable = [ ... ];
public function posts()
{
return $this->belongsToMany('App\Post', 'post_tag', 'tag_id', 'post_id');
}
}
Thanks for yours anser!

Answer 1

The error states that you're calling sync() on a null object, which means the result of your tags() method is null.
If you take a look at your tags() method, you can see that you forgot to return the relationship, therefore it is returning null. Add the return keyword and you should be good.
public function tags()
{
return $this->belongsToMany('App\Tag', 'post_tag', 'post_id', 'tag_id');
}

Answer 2

Try this
// store post in the database
$post = new Post([
'title' => $request->title,
'body' => $request->body,
]);
$post->save();
$post->tags()->sync([1, 2, 3], false);