Laravel 5 : Many-to-Many Relationships, article_id always null "SQLSTATE[23000]: Integrity constraint violation: 1048"

I'm new for laravel, I try to learn from http://ift.tt/1LRcx0L

when I run from cmd php artisan tinker -> $article->tags()->attach(1)

get an error:

Illuminate\Database\QueryException with message 'SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'article_id' cannot be null (SQL: insert into `article_tag` (`article_id`, `tag_id`) values (, 1))'

article_id value is NULL. I don't understand why article_id always null??

Article.php (app\Article.php):

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Article extends Model
{
    protected $fillable = array('title', 'body', 'published_at');

    /*
    * Get the tags associated with the given article
    */
    public function tags(){
        return $this->belongsToMany('App\Tag');
    }

}

Tag.php (app/Tag.php):

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Tag extends Model
{
    protected $fillable = array('name');

    /*
    * Get the articles associated with the given tag
    */
    public function articles(){
        return $this->belongsToMany('App\Article');
    }
}

migration create_articles_table:

<?php

use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Migrations\Migration;

class CreateArticlesTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('articles', function (Blueprint $table) {
            $table->increments('id');
            $table->string('title');
            $table->text('body');
            $table->dateTime('published_at');
            $table->timestamps();
        });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::drop('articles');
    }
}

migration create tags and article_tag:

<?php

use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Migrations\Migration;

class CreateTagsTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('tags', function (Blueprint $table) {
            $table->increments('id');
            $table->string('name', 255);
            $table->timestamps();
        });

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

            $table->integer('tag_id')->unsigned()->index();
            $table->foreign('tag_id')->references('id')->on('tags')->onDelete('cascade');

            $table->timestamps();
        });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::drop('tags');
        Schema::drop('article_tag');
    }
}

and I don't know where is my mistake. Thank's for your help..

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