A helper Trait for Laravel Eloquent Models - https://metricloop.com/blog/how-to-let-your-laravel-eloquent-models-find-themselves?utm_source=github_gist&utm_medium=blog&utm_content=post_17&utm_campaign=github_gists

CanResolveSelf.php

<?php

namespace App\Traits;

use Illuminate\Database\Eloquent\ModelNotFoundException;

trait CanResolveSelf
{
    /**
     * Resolves model regardless of given identifier.
     *
     * @param $model
     * @param bool $withTrashed
     * @return mixed
     * @throws \Exception
     */
    public static function resolveSelf($model, $withTrashed = false)
    {
        $className = get_called_class();

        if(is_null($model)) {
            return null;
        }
        
        if(!$model instanceof $className) {
            if(is_numeric($model)) {
                try {
                    $model = $className::when($withTrashed, function ($query) {
                        return $query->withTrashed();
                    })->findOrFail($model);
                } catch (ModelNotFoundException $e) {
                    throw new \Exception($className . ' not found with the given ID.');
                }
            } else {
                try {
                    $model = $className::when($withTrashed, function ($query) {
                        return $query->withTrashed();
                    })->where('slug', $model)->firstOrFail();
                } catch (ModelNotFoundException $e) {
                    throw new \Exception($className . ' not found with the given slug.');
                }
            }
        }
        return $model;
    }
}

CanResolveSelfTraitTest.php

<?php

use Illuminate\Foundation\Testing\DatabaseMigrations;

class CanResolveSelfTraitTest extends TestCase
{
    use DatabaseMigrations;

    /** @test */
    public function if_given_itself_should_return_itself()
    {
        $posts = factory(App\Post::class, 2)->create();
        $post = App\Post::findOrFail(1);
        $result = App\Post::resolveSelf($post);
        $this->assertEquals($post, $result);
    }

    /** @test */
    public function if_given_null_should_return_null()
    {
        factory(App\Post::class, 2)->create();
        $result = App\Post::resolveSelf(null);
        $this->assertNull($result);
    }

    /** @test */
    public function if_given_id_should_return_appropriate_model()
    {
        factory(App\Post::class, 2)->create();
        $post = App\Post::findOrFail(1);
        $result = App\Post::resolveSelf(1);
        $this->assertEquals($post, $result);
    }

    /** @test */
    public function if_given_text_should_return_appropriate_model()
    {
        factory(App\Post::class, 2)->create();
        $post = App\Post::findOrFail(1);
        $result = App\Post::resolveSelf($post->slug);
        $this->assertEquals($post, $result);
    }

    /** 
     * @test
     * @expectedException \Exception
     * @expectedExceptionMessage App\Post not found with the given ID.
     */
    public function should_throw_exception_if_id_not_found()
    {
        factory(App\Post::class, 2)->create();
        App\Post::resolveSelf(3);
    }

    /**
     * @test
     * @expectedException \Exception
     * @expectedExceptionMessage App\Post not found with the given slug.
     */
    public function should_throw_exception_if_slug_not_found()
    {
        factory(App\Post::class, 2)->create();
        App\Post::resolveSelf('foo');
    }

    /** @test */
    public function returns_appropriate_model_from_id_even_if_it_was_soft_deleted()
    {
        factory(App\Post::class, 2)->create();
        $post = App\Post::findOrFail(1);
        $post->delete();
        $result = App\Post::resolveSelf(1, $withTrashed = true);
        $this->assertEquals(App\Post::withTrashed()->findOrFail(1), $result);
        $this->assertTrue($result->trashed());
    }

    /** @test */
    public function returns_appropriate_model_from_slug_even_if_it_was_soft_deleted()
    {
        factory(App\Post::class, 2)->create();
        $post = App\Post::findOrFail(1);
        $post->delete();
        $result = App\Post::resolveSelf($post->slug, $withTrashed = true);
        $this->assertEquals(App\Post::withTrashed()->findOrFail(1), $result);
        $this->assertTrue($result->trashed());
    }

}