Pass Variable from Component to Model Scope

I'm using OctoberCMS based on Laravel.

I'm trying to get the identifier from URL and pass it to the Scope to filter database results.

$this->property('username') works and returns the username from the URL.

But how do you pass it to the Model and into the Scope function?

Here is the guide, under Dynamic Scopes.

https://octobercms.com/docs/database/model#query-scopes

Page

URL: localhost/user/matt

Identifier: /user/:username

Results Component

public function init()
{
    // get username from url
    $username = $this->property('username'); //matt

    // pass username to scope
    Gallery::applyUser($username);
}

Gallery Model

// return results that match username
public function scopeApplyUser($query, $username)
{
    return $query->where('username', $username);
}

Error

Missing argument 2 for MyVendor\Gallery\Models\Gallery::scopeApplyUser()

Solution?

I found adding ($query, $username = null) allows the variable to pass without error.

But now the problem is that $username is both 'matt' and null at the same time and never makes it to the query return.

// return results that match username
public function scopeApplyUser($query, $username = null)
{
    print $username; //prints matt
    if ($username == null) { print 'null'; } //prints null

    return $query->where('username', $username); //returns null
}

In a model gallery you need a field user:

class Gallery extends Model {
    /** these are just placeholder variables you should be recognising these from your own model.!**/
    protected $table = 'gallery';
    protected $guarded = ['*'];
    protected $fillable = ['username'];// Make sure this also is in the database!!
    public scopeWhereUser($query, $user = null) {
        if(!is_null($user)) { // Only do something if the username is provided!
           $query->where('username',$user);
        }
    }
}

Then, when you have the nullchecks you can just call it with

 $gallery = new Gallery();
 $query = $gallery->newQuery()->where('x','y')->whereUser('matt');

Changes I made:

renamed scope to whereuser instead of apply user because it's more logical to name it that way. Apply you do to something functional that changes a state permanently
added a not is_null() check to only limit the query when it's not null.

full working example that prints the results:

 $gallery = new Gallery();
 $query = $gallery->newQuery()->where('x','y')->whereUser('matt');
 $results = $query->get();
 foreach($results as $galleryItem) {
     echo $galleryItem->getKey() . ' is having:<BR/>';
     echo dump([
               'model'=>$galleryItem,
               'methods' => get_class_methods(get_class($galleryItem)),
               ]);
 }