Call to a member function comments() on null when use foreign key - laravel

Question

This is error Call to a member function comments() on null, in model give a relation still show this error
Article model
function user() {
return $this->belongsTo(User::class, 'id');
}
public function comments()
{
return $this->hasMany(Comment::class, 'id');
}
Comment Model
public function user()
{
return $this->belongsTo(User::class);
}
public function article()
{
return $this->belongsTo('App\Article');
}
This is contoller
public function store(Request $request)
{
//dd($request->all());
Auth::user();
$comment = new Comment;
$comment -> user_id=Auth::user()->id;
$comment-> comment = $request->get('comment');
$article = Article::find($request->article_id);
$article->comments()->save($comment);
return redirect()->back()->with('success', 'your comment,successfully save');
}
This is blade file
<form method="post" action="{{ route('comment.store') }}">
#csrf
<div class="form-group">
<textarea class="form-control" name="comment"></textarea>
<input type="hidden" name="article_id"/>
</div>
<div class="form-group">
<input type="submit" class="btn btn-success" value="Add Comment" />
</div>
</form>

Answer 1

You need to set a value for the hidden field.
<input type="hidden" name="article_id" value="{{ $article->id }}"/>
Secondly for easier debugging these errors, using findOrFail() will ensure your app blows up, with a proper error message. Instead of just returning null.
$article = Article::findOrFail($request->article_id);
EDIT
You are also mixing two saving approaches together, either you associate the article to the comment or create a new comment. This is the approach i like to use.
$comment = new Comment;
$comment->user_id = Auth::user()->id;
$comment->comment = $request->get('comment');
$article = Article::find($request->article_id);
$comment->article()->associate($article);
$comment->save();