Survey structure

Have you watched some basic laracast on relations? Plus there's examples and a tutorial in the docs.

Classes for?

Deberías ver los vídeos básicos de laravel, ahí entenderás muchas cosas. https://laracasts.com/series/laravel-5-from-scratch

I watched, but I'm trying to construct classes for manage some behavior of the surveys and questions, I don't want to put this behavior in Eloquent models. Instead I think my classes should use Eloquent to save the data to the database. But I'm not sure, I find myself constructing methods like create(), find(), saveExisting(), saveNew() inside my objects and in this methods using eloquent to save data or for retrieve data and construct the object of a existing row in the db. I'm not sure if this methods should be there. For example I have my base class like this

abstract class Question
{
    protected $survey;
    protected $id = NULL;
    protected $text;

    protected $surveyModel;

    /**
     * Class Constructor
     * @param    $text   
     * @param    $answer   
     */
    protected function __construct(Survey $survey, $id, $text)
    {
        $this->survey = $survey;
        $this->surveyModel = SurveyModel::find($survey->id);
        $this->id = $id;
        $this->text = $text;
    }

    public static function create(Survey $survey, $text)
    {
        return (new static($survey, NULL, $text))->save();
    }

    public static function find($id)
    {
        $existingQuestion = QuestionModel::find($id);
        $survey = Survey::find($existingQuestion->survey_id);
        return new static($survey, $existingQuestion->id, $existingQuestion->text);
    }

    public function save()
    {
        if (is_null($this->id)) 
        {
            return $this->saveNew();
        }

        return $this->saveExisting();
    }

    protected function saveNew()
    {
        $this->surveyModel->questions()->create([
            'text'  => $this->text
        ]);
        return $this;
    }

    protected function saveExisting()
    {
        $existingQuestion = QuestionModel::find($this->id);
        $existingQuestion->update([
            'text' => $this->text
        ]);
        $this->id = $existingQuestion->id;
        return $this;
    }


    /**
     * Allow for property-style retrieval
     *
     * @param $property
     * @return mixed
     */
    public function __get($property)
    {
        if (property_exists($this, $property))
        {
            return $this->{$property};
        }

        return false;
    }

}

I think you're going into abstraction way to early in the process. Try using only 2 Eloquent models first instead of your own classes (you can refactor later if needed):

Class Survey extends Model {
    ...
    ...
    public function questions() {
        return $this->hasMany(Question::class);
    }
}

Class Question extends Model {
    ...
    ...
    public function survey() {
        return $this->belongsTo(Survey::class);
    }   
}

Now you can already do:

$survey = new Survey();
$survey->name = 'test';
$survey->save();

$question = new Question();
$question->text = 'What I want to ask';

$survey->questions()->save($question);

# or

# Give me survey 2 with all it's questions
$survey = Survey::with('questions')->find(2);
foreach($survey->questions as $question) {
    echo $question->text .'<br />';
}

Each model is responsible for only 1 thing: communicate with the storage engine Now you can give the question model a 'type' field and perhaps 'options' for the dropdown/checkbox/radio questions and you could create a few small classes (1 for each type of question) to actually render the input field.

# QuestionModel:
public function render() {
    // $this->type can be  text, date, email, select etc.
    // App::make will throw an exception if trying to load a class that doesnt exist
    $renderClass = \App::make('App\\Presenters\\Questions\\'. ucfirst($this->type));
    return $renderClass->render($this);
}

# App\Presenters\Questions\Text.php:
public function render(Question $question) {
    return view('inputs.text', $question);
}

# App\Presenters\Questions\Date.php:
public function render(Question $question) {
    return view('inputs.date', $question);
}
# etc.

This way all classes will adhere to the Single Responsibility Pattern)