Why is my form not posting to the pivot table?

I am using Laravel 5. I have been following this tutorial about using a form to create an article with 'tags'. I have replaced tags with categories. My articles 'belong to many' categories and my categories 'belong to many' articles. I have a form to create a new article but it only creates an entry in the articles table and not in the 'article_category' pivot table.

I get this error: "FatalErrorException in ArticlesController.php line 77: Call to a member function categories() on array". I think the problem is with fourth line in the 'store' function. I can't find anyone with the same problem. That line is supposed to attach the article id to the category ids but it's not working. Grateful for any help.

My ArticlesController:

public function store(CreateArticleRequest $request)
{

     $article = $request->all();

    Article::create($article);

    $categoriesId = $request->input('categories');

    $article->categories()->attach($categoriesId);

    return redirect()->route('articles_path');

}

The namespaces:

namespace App\Http\Controllers;

use App\Article;
use Illuminate\Http\Request;
use App\Http\Requests;
use App\Http\Requests\CreateArticleRequest;
use App\Http\Controllers\Controller;
use App\Category;
use App\Day;

Routes:

Route::resource('articles', 'ArticlesController', [

    'names' => [

        'index' => 'articles_path',
        'show' => 'article_path',
        'edit' => 'articleEdit_path',
        'update' => 'articleUpdate_path',
        'create' => 'articleCreate_path',
        'store' => 'articleStore_path'
    ]

    ]);

Article Model:

namespace App;
use Illuminate\Database\Eloquent\Model;
class Article extends Model
{
    public function categories()
    {
        return $this->belongsToMany('App\Category', 'article_category', 'category_id', 'article_id')->withTimestamps();
    }
protected $fillable = array('title', 'description', 'image', 'lat', 'lng');

}

Category Model

namespace App;

use Illuminate\Database\Eloquent\Model;

class Category extends Model
{
    public function articles()
    {
        return $this->belongsToMany('App\Article', 'article_category', 'article_id', 'category_id')->withTimestamps();
    }

protected $fillable = array('name'); 
}

PivotTable:

Schema::create('article_category', function (Blueprint $table) {
    $table->integer('article_id')->unsigned()->index();
    $table->foreign('article_id')->references('id')->on('articles')->onDelete('cascade');
    $table->integer('category_id')->unsigned()->index();
    $table->foreign('category_id')->references('id')->on('categories')->onDelete('cascade');
    $table->timestamps();
});

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