Stuck at storing related records with bidirectional relations

Hello,

I'm currently stuck at storing related records and I'll be glad for every idea that might help solve my issue.

The logic behind this is almost self-explaining:
Users can send in questions which can be answered by experts.
Users <> Questions <> Answers <> Experts

Users.php

<?php
class Users
{
    public $id;
    public $name;
    public $email;

    // Initialize
    public function initialize()
    {
        // Establish relation to questions model
        $this->hasMany('id', 'Questions', 'userId', array(
            'alias' => 'questions'  
        ));

    }

}

Questions.php

<?php
class Questions
{
    public $id;
    public $userId;
    public $text;

    // Initialize
    public function initialize()
    {
        // Establish relation to users model
        $this->belongsTo('userId', 'Users', 'id');
        $this->hasOne('userId', 'Users', 'id', array(
            'alias' => 'user'
        ));

        // Establish relation to answers model
        $this->belongsTo('id', 'Answers', 'questionId');
        $this->hasOne('id', 'Answers', 'questionId', array(
            'alias' => 'answer'
        ));

    }

}

Answers.php

<?php
class Answers
{
    public $id;
    public $questionId;
    public $expertId;
    public $text;

    // Initialize
    public function initialize()
    {
        // Establish relation to experts model
        $this->belongsTo('expertId', 'Experts', 'id');
        $this->hasOne('expertId', 'Experts', 'id', array(
            'alias' => 'expert'
        ));

        // Establish relation to questions model
        $this->belongsTo('questionId', 'Questions', 'id');
        $this->hasOne('questionId', 'Questions', 'id', array(
            'alias' => 'question'
        ));

    }

}

Experts.php

<?php
class Experts
{
    public $id;
    public $firstName;
    public $lastName;
    public $nickname;
    public $email;

    // Initialize
    public function initialize()
    {
        // Establish relation to Questions model
        $this->hasMany('id', 'Answers', 'expertId', array(
            'alias' => 'answers'
        ));

    }

}

The following code is adapted from the documentation found at https://docs.phalcon.io/en/latest/reference/models.html#storing-related-records
but obviously doesn't work…

<?php
$user = Users::findFirstById(1);

$question = new Questions();
$question->text = 'Lorem ipsum';
$question->user = $user;

if (!$question->save())
{
    foreach($question->getMessages() as $message)
    {
        echo $message->getMessage() . '<br>';

    }

}

Instead it returns the following message:

userId is required

Same happens If I try to save other related objects and the worst thing I found out is, that the following lines both store new records for Experts as well as Questions instead of referencing the relations as virtual foreign keys:

<?php
$question  = Questions::findFirstById(1);
$expert = Experts::findFirstById(1);

$answer = new Answer();
$answer->question = $question;
$answer->expert = $expert;
$answer->text = 'Hello world';

if (!$answer->save())
{
    foreach($answer->getMessages() as $message)
    {
        echo $message->getMessage() . '<br>';

    }

}

Any ideas of what's wrong with my code?

Thanks,
Cedric