Symfony Form customize array for entity type

In my Symfony2.6.6 project I have a category entity. I don't know how to say that properly but use the doctrine tree extension to create a tree with the category entity like:

• Category 1
  
  
  
  • Child Category 1
  
  
  • Child Category 2
  
  
  
  


• Category 2
  
  
  
  • Child Category 3 ...
  
  
  
  

Now when creating the form type for creating a new Category I want to customize the array of the parent field.

The code I use

$builder->add('parent', 'entity', array('class' => 'AcmeBlogBundle:Category', 'property' => 'title'));

generates just an array with all the title as expected. But I want the array values to append a '-' for every level of the tree. Category 1 for example would be '- Category 1' and Child Category 1 would be '-- Child Category 1'. How can I do that?

Sorry if this has been asked before, I don't know how to search for that.