Влад

Думаю вы просто перепутали аргументы:
public function document_types()
{
    return $this->hasOne(DocumentType::class, 'id', 'type_id');
}

$user = User::where('id', '1')->first();
foreach ($user->roles as $role) {
    echo $role->name;
}

@if ($loop->last)
    This is the last iteration.
@endif

@if ($loop->index % 2 == 0)
    Четное
@else
    Нет
@endif

public function auth()
    {
        // Authentication Routes...
        $this->get('login', 'Auth\LoginController@showLoginForm')->name('login');
        $this->post('login', 'Auth\LoginController@login');
        $this->post('logout', 'Auth\LoginController@logout')->name('logout');

        // Registration Routes...
        $this->get('register', 'Auth\RegisterController@showRegistrationForm')->name('register');
        $this->post('register', 'Auth\RegisterController@register');

        // Password Reset Routes...
        $this->get('password/reset', 'Auth\ForgotPasswordController@showLinkRequestForm')->name('password.request');
        $this->post('password/email', 'Auth\ForgotPasswordController@sendResetLinkEmail')->name('password.email');
        $this->get('password/reset/{token}', 'Auth\ResetPasswordController@showResetForm')->name('password.reset');
        $this->post('password/reset', 'Auth\ResetPasswordController@reset');
    }

public function get{Свойство}Attribute($value)
{
    return is_null($value) ? '' : $value;
}

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

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

protected $table = 'categories';
public function posts()
{
    return $this->belongsToMany('App\Models\Post', 'post_id', 'category_id);
}

protected $table = 'posts';
public function categories()
{
    return $this->belongsToMany('App\Models\Category', 'post_id', 'category_id);
}

$post = Post::where('slug', $slug)->first();  -----> равенство идет по дефолту
$categories = $post->categories;
foreach ($categories as $category)
{
    echo $category->id. '<br>';   -----------------> любые данные категории.
}