Laravel5: QueryException while writing data to related database table. Error: "Cannot add or update a child row: a foreign key constraint fails"

Question

I just created a categories table and related it to articles table.

$table->foreign('category_id')->references('id')->on('categories');

I also created a category selection field with name 'category_id' in the 'Create Article' form. Now while trying to create a new article, after I hit the submit button, I get the following error:

QueryException in Connection.php line 725: SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (laratry.articles, CONSTRAINT articles_category_id_foreign FOREIGN KEY (category_id) REFERENCES categories (id)) (SQL: insert into articles (title, body, published_at, user_id, updated_at, created_at) values (This is a test article, This is yet another test article for trying to find what the error is., 2016-08-15 16:11:53, 1, 2016-08-15 16:11:53, 2016-08-15 16:11:53))

If I print_r( $request->all() ),

Array ( 
    [_token] => E4fWVTpf2T5y23uOnaA0EK1hHyi2hYvkJZa99V3G 
    [title] => This is a test article 
    [body] => This is yet another test article for trying to find what the error is. 
    [category_id] => 1 
    [published_at] => 2016-08-15
    [tag_list] => Array ( [0] => 1 )
)

My Controller's store method for article:

class ArticlesController extends Controller
{
    public function store(Request $request)
    {
        Auth::user()->articles()->create($request->all());
        flash()->success('Your article has been created.');
        return redirect('articles');
    }
}

My Models:

class Article extends Model
{
    protected $fillable = ['title','body','published_at','user_id'];

    protected $dates = ['published_at','created_at'];

    public function category()
    {
        return $this->belongsTo('App\Category');
    }
}

--

class Category extends Model
{
    protected $fillable = ['name','slug'];
    protected $dates = ['created_at'];

    public function articles()
    {
        return $this->hasMany('App\Article');
    }
}

Answer 1

You miss the category_id.
Change

protected $fillable = ['title','body','published_at','user_id'];

To

protected $fillable = ['title','body','published_at','user_id', 'category_id'];