$builder
            ->add('son', ModelType::class, [
                'label' => 'labels.title',
                'query' => MySonObjectQuery::create(),
                'expanded' => true,
                'class' => MySonObject::class,
                'constraints' => [
                    new NotBlank([
                        'message' => 'form.error.not_blank',
                    ]),
                ],
            ]);

    <table name="my_object">
        <column name="id" type="INTEGER" required="true" primaryKey="true" autoIncrement="true" />
        <column name="son_id" type="INTEGER" size="10" required="true" />

        <foreign-key foreignTable="my_son_object" phpName="Son">
            <reference local="son_id" foreign="id" />
        </foreign-key>
    </table>

    <table name="my_son_object">
        <column name="id" type="INTEGER" required="true" primaryKey="true" autoIncrement="true" />
        <column name="name" type="VARCHAR" size="10" required="true" />
    </table>

[1] Symfony\Component\Debug\Exception\ContextErrorException: Warning: call_user_func() expects parameter 1 to be a valid callback, first array member is not a valid class name or object
    at n/a
        in /home/peter/io/symfony_base_project/vendor/propel/propel-bundle/Form/Type/ModelType.php line 181

    at Symfony\Component\Debug\ErrorHandler->handleError('2', 'call_user_func() expects parameter 1 to be a valid callback, first array member is not a valid class name or object', '/home/peter/io/symfony_base_project/vendor/propel/propel-bundle/Form/Type/ModelType.php', '181', array('object' => null, 'firstIdentifier' => object(ColumnMap)))
        in  line 

    at call_user_func(array(null, 'getId'))
        in /home/peter/io/symfony_base_project/vendor/propel/propel-bundle/Form/Type/ModelType.php line 181

    at Propel\Bundle\PropelBundle\Form\Type\ModelType->Propel\Bundle\PropelBundle\Form\Type\{closure}(null)
        in  line 

    at call_user_func(object(Closure), null)
        in /home/peter/io/symfony_base_project/vendor/symfony/symfony/src/Symfony/Component/Form/ChoiceList/ArrayChoiceList.php line 158

    at Symfony\Component\Form\ChoiceList\ArrayChoiceList->getValuesForChoices(array(null))
        in /home/peter/io/symfony_base_project/vendor/symfony/symfony/src/Symfony/Component/Form/ChoiceList/LazyChoiceList.php line 198

    at Symfony\Component\Form\ChoiceList\LazyChoiceList->getValuesForChoices(array(null))
        in /home/peter/io/symfony_base_project/vendor/symfony/symfony/src/Symfony/Component/Form/Extension/Core/DataTransformer/ChoiceToValueTransformer.php line 37

    at Symfony\Component\Form\Extension\Core\DataTransformer\ChoiceToValueTransformer->transform(null)
        in /home/peter/io/symfony_base_project/vendor/symfony/symfony/src/Symfony/Component/Form/Form.php line 1092

    at Symfony\Component\Form\Form->normToView(null)
        in /home/peter/io/symfony_base_project/vendor/symfony/symfony/src/Symfony/Component/Form/Form.php line 352

    at Symfony\Component\Form\Form->setData(null)
        in /home/peter/io/symfony_base_project/vendor/symfony/symfony/src/Symfony/Component/Form/Extension/Core/DataMapper/PropertyPathMapper.php line 57

    at Symfony\Component\Form\Extension\Core\DataMapper\PropertyPathMapper->mapDataToForms(object(MyObject), object(RecursiveIteratorIterator))
        in /home/peter/io/symfony_base_project/vendor/symfony/symfony/src/Symfony/Component/Form/Form.php line 385

    at Symfony\Component\Form\Form->setData(object(MyObject))
        in /home/peter/io/symfony_base_project/vendor/symfony/symfony/src/Symfony/Component/Form/Form.php line 477

    at Symfony\Component\Form\Form->initialize()
        in /home/peter/io/symfony_base_project/vendor/symfony/symfony/src/Symfony/Component/Form/FormBuilder.php line 226

    at Symfony\Component\Form\FormBuilder->getForm()
        in /home/peter/io/symfony_base_project/vendor/symfony/symfony/src/Symfony/Component/Form/FormFactory.php line 39

    at Symfony\Component\Form\FormFactory->create('AppBundle\Form\MyObjectType', object(MyObject), array('action' => '/app_dev.php/test'))
        in /home/peter/io/symfony_base_project/vendor/symfony/symfony/src/Symfony/Bundle/FrameworkBundle/Controller/Controller.php line 309

    at Symfony\Bundle\FrameworkBundle\Controller\Controller->createForm('AppBundle\Form\MyObjectType', object(MyObject), array('action' => '/app_dev.php/test'))
        in /home/peter/io/symfony_base_project/src/AdminBundle/Controller/DefaultController.php line 28

    at AdminBundle\Controller\DefaultController->testAction(object(Request))
        in  line 

    at call_user_func_array(array(object(DefaultController), 'testAction'), array(object(Request)))
        in /home/peter/io/symfony_base_project/vendor/symfony/symfony/src/Symfony/Component/HttpKernel/HttpKernel.php line 153

    at Symfony\Component\HttpKernel\HttpKernel->handleRaw(object(Request), '1')
        in /home/peter/io/symfony_base_project/vendor/symfony/symfony/src/Symfony/Component/HttpKernel/HttpKernel.php line 68

    at Symfony\Component\HttpKernel\HttpKernel->handle(object(Request), '1', true)
        in /home/peter/io/symfony_base_project/vendor/symfony/symfony/src/Symfony/Component/HttpKernel/Kernel.php line 169

    at Symfony\Component\HttpKernel\Kernel->handle(object(Request))
        in /home/peter/io/symfony_base_project/web/app_dev.php line 30