A test model used for the relationship tests.
Inheritance: extends Cviebrock\EloquentSluggable\Tests\Models\Post
 /**
  * Test generating slug when related model doesn't exists.
  *
  * @test
  */
 public function testSlugFromRelatedModelNotExists()
 {
     $post = PostWithRelation::create(['title' => 'First']);
     $this->assertEquals('first', $post->slug);
 }