Update a record with a single field

I'm using the l5-repository in my project, but I'm having the following difficulty:

I have the code below to make the slug unique, but when I do an update it says that slug already exists, in> slug of the record I'm editing. How can I resolve this?

class LandingValidator extends LaravelValidator
{
    protected $rules = [
        ValidatorInterface::RULE_CREATE => [
            'slug' => 'required|unique:landings,slug',
        ],
        ValidatorInterface::RULE_UPDATE => [
            'slug' => 'required|unique:landings,slug',
        ],
   ];
}

I tried this way, but it did not work:

ValidatorInterface::RULE_UPDATE => [
    'slug' => 'required|unique:landings,slug,'.$landing->id,
],

  

FatalErrorException in LandingValidator.php line 25:

     

syntax error, unexpected '$ landing' (T_VARIABLE)