Problem with removing all related records

I have 2 models that handle sliders and slides for my website. Blow is a small part of the used code. This is inside a wordpress backend, using the phalcon micro framework.

  // Existing or new image ?
  $id =$app->request->getPost( 'id', 'int' );
  if( !is_numeric( $id ) && $id <= 0 ) $slider =new Sliders();
  else                                 $slider =Sliders::findFirst( $id );

  $slider->cover =Images::findFirst( $app->request->getPost( 'cover', 'int' ));

  $slider->setName( $app->request->getPost( 'name', 'string' ));
  $slider->setDescription( $app->request->getPost( 'description' ));

  $slides =[];
  foreach( $app->request->getPost( 'slides' ) as $slide )
  {
    $dummy =new Slides();

    $dummy->image =Images::findFirst( $slide['image'] );

    $dummy->setLink( $slide['link'] );
    $dummy->setActive( $slide['active'] );
    $dummy->setCaption( $slide['caption'] );
    $dummy->setExternal( $slide['external'] );

    $slides[] =$dummy;
  }

  // Remove all old slides
  $dummy =Slides::find([
    'conditions' => 'slider_id = ?1',
    'bind' => [
      '1' => $slider->getId()
    ]
  ]);
  $dummy->delete();

  // Add the new slides
  $slider->slides =$slides;

  // Create or update region
  if( $slider->save())

The above code is used to save a slider and its slides. Although I had a strange thing happening when using something else to delete the former slides. The code below sees to be the solution.

  // Remove all old slides
  $dummy =Slides::find([
    'conditions' => 'slider_id = ?1',
    'bind' => [
      '1' => $slider->getId()
    ]
  ]);
  $dummy->delete();

But I don't get it why the code below isn't working as expected. When I use the code below instead of the fin above. All the old slides get removed, but the new slides don't get added. I know the relations on the two modes are correct.

$slider->slides->delete();