Laravel unique request update

Ive read some things about this on laracasts and Stackoverflow.
I have an update function with validation:
public function update(Customer $customer, StoreCustomer $request)
{
    $customer->update($request->validated());
    exit();
}
And the validation rules:
 public function rules()
{
    return [
        'code'  => 'required|unique:customers,code',
    ]
}
Now I tried to add a 3rd argument after the unique, so if it would exist it would continue. I tried it like this:
 public function rules(Customer $customer)
 {
    return [
        'code'  => 'required|unique:customers,code,'.$customer->code,
    ]
}
but that doesn't seem to do anything. It seems to work if you do the validation in my controller itself, but this looks way cleaner. Any solutions?