[5.4] add getMorphedModel to Relation class

[Lloople]

I couldn't found a way to do it with the current classes or methods. What this method does is to retrieve the model class that will be used before a related model is created. It's useful for example like this:

// app/Providers/AppServiceProvider.php

Relation::morphMap([
    'posts' => 'App\Post',
    'videos' => 'App\Video',
]);

// any place on your application

$morphedClass = Relation::getMorphedModel($comment->commentable_type); // posts to App\Post

$morphedModel = $morphedClass::createFromComment($comment); // did this way because Post and Video have different initial/default values

[tomlankhorst]

The inverse:

Relation::macro('morphAlias', function (string $class) {
  return array_search($class, Relation::$morphMap) ?: $class;
});```

[ohnotnow]

@tomlankhorst that's just saved me a lot of tinkering - thanks! :-)

[Lloople]

@tomlankhorst It would be nice to have that on the framework by default. Have you tried creating a Pull Request?