Override eloquent's save method on pivot table

I have three tables: Products, Categories and a pivot one CategoryProduct.

I have an attribute updated_by and I want this to be automatically set to Auth::id() upon all create/update requests.

So far I am able to save data by using the sync() method ie.

$categoriesArray = [ 
    1 => ['updated_by' => Auth::id()],
    2 => ['updated_by' => Auth::id()],
    ...
];
$product->categories()->sync($categoriesArray); 

I want to simplify the above by overriding the save method on CategoryProduct model ie.

class CategoryProduct extends Model
{

    protected $table = 'category_product';
    protected $fillable = ['product_id', 'category_id', 'created_by', 'updated_by'];

    public function save(array $options = array())
    {
        $this->updated_by = $this->updated_by ?: Auth::id();

        parent::save($options);
    }
}

So that I can save my categories like so:

$categoriesArray = [1, 2, 3];
$product->categories()->sync($categoriesArray); 

However for some reason the save method is not triggered upon saving resulting in a MySQL error. General error: 1364 Field 'updated_by' doesn't have a default value.

Any ideas?