Doctrine MongoDB with Inheritance not persisting fields

In order to better isolate the problem i tried to simplify my code, here it is:

This Document Trait is used as a 'generic' mongo document. I basically don't want to be rewritting $id and $createdAt in every document

/**
 * Class Document
 * @package TMPBundle\MongoDocument
 * @MongoDB\MappedSuperclass()
 */
trait Document {
    /**
     * @MongoDB\Id()
     */
    protected $id;
    /**
     * @MongoDB\Date()
     */
    protected $createdAt;
}

Now the problem:

This class represents a generic element(a question) in a survey.The only data i need to persist is on this class ( but in the future i may need to add data to subclasses)

   /**
     * @MongoDB\MappedSuperclass()
     * @MongoDB\InheritanceType("SINGLE_COLLECTION")
     * @MongoDB\DiscriminatorField(fieldName="type")
     * @MongoDB\Collection(name="survey_elements")
     */
    abstract class AbstractSurveyType {
        use Document;

        /**
         * @MongoDB\String()
         */
        public $question;
        /**
         * @MongoDB\Collection()
         */
        public $value;
        /**
         * @MongoDB\Bool()
         */
        public $required;
        /**
         * @MongoDB\Bool()
         */
        public $hasRemark;
        /**
         * @MongoDB\String()
         */
        public $remarkQuestion;

    }

This class represents a question that only has one choice as an answer ( a text input, a file input, etc).

/**
 * Class SingleSurveyType
 * @package TMPBundle\MongoDocument\Survey\SurveyTypes\SingleSurveyType
 * @MongoDB\Document()
 */
abstract class SingleSurveyType extends AbstractSurveyType{

    public function validate(){
        if (count($this->value) != 1){
            throw new InvalidSurveyElement("A SingleSurveyType cannot have more than one available value");
        }
    }
}

And finally i have my question with some extra methods:

  /**
 * Class FileSurveyType
 * @package TMPBundle\MongoDocument\Survey\SurveyTypes\SingleSurveyType\Types
 * @MongoDB\Document()
 */
class FileSurveyType extends SingleSurveyType{

    protected function getFieldName()
    {
        return 'survey_question_file';
    }

    protected function getFieldType()
    {
        return 'file';
    }

}

In my controller when i'm just doing this:

   public function submitSurveyAction(Request $request){

        $file = new FileSurveyType();
        $file->setValue(["algo"]);
        $file->setQuestion("questao");

        $this->get('doctrine_mongodb.odm.document_manager')->persist($file);
        $this->get('doctrine_mongodb.odm.document_manager')->flush();
}

After running the controller Action the desired fields but if i set the properties in AbstractSurveyType as protected the fields are not persisted. Also, after running ´php app/console doctrine:mongodb:generate:documents´ for the first time (with public modified) i get this error:

´Access level to SingleSurveyType::$question must be public (as in class AbstractSurveyType)

My Initial Question:

I'm currently using doctrine in symfony2 to persist data to a MongoDB database. my project is just a form/survey creation system.I have several question types , each question type can be a singleType (text,number) or multipleType (choices, checkboxes,etc). I'm trying to save some documents using inheritance. This is a small set of what i have:

trait Document {
/**
 * @MongoDB\Id()
 */
protected $id;
/**
 * @MongoDB\Date()
 */
protected $createdAt;
}

Then i use this Document in all documents ( so that i don't have to keep repeating the id property and the updatedAt )

 * @MongoDB\MappedSuperclass()
 * @MongoDB\DiscriminatorField("type")
 * @MongoDB\InheritanceType("SINGLE_COLLECTION")
 */

abstract class AbstractSurveyType {
    /**
     * @MongoDB\String()
     */
    protected $question;
}

Before reaching the final class i still have this one with only a couple of methods ( single and multiple types are not handled in the same way)

/**
 * Class SingleSurveyType
 * @MongoDB\MappedSuperclass()
 * @MongoDB\InheritanceType("SINGLE_COLLECTION")
 */
abstract class SingleSurveyType extends AbstractSurveyType{
}

And finally i have my type:

/**
 * @MongoDB\Document()
 * @package TMPBundle\MongoDocument\Survey\SurveyTypes\SingleSurveyType\Types
 */
class NumberSurveyType extends SingleSurveyType{
use Document;
}

The problem i'm having is that when i persist the data to the database the properties inherited from Document, AbstracSurveyType and SingleSurveyType do not get persisted. My documents only get a _id and type properties... Is this supposed to happend?

Thanks in advance :)