Since 2.7 using expanded Entity Form Elements, which allow multiple selections (checkboxes) with data having negativ IDs is not possible anymore, because the new naming conventions prohibit names starting with a '-'

Example

Database Table

Code

class ExampleForm extends Form
{
    public function buildForm(FormBuilderInterface $builder, array $options) {
        $builder->add(
            'element',
            'entity',
            array(
                'class' => 'AppBundle:ExampleEntity',
                'label' => 'Expanded Entity',
                'expanded' => true,
                'multiple' => true,
            )
        );
    }
}

Creating this form with the Symfony Form Builder will generate the following Error:

The name "-1" contains illegal characters. Names should start with a letter, digit or underscore and only contain letters, digits, numbers, underscores ("_"), hyphens ("-") and colons (":").

Prior to Symfony 2.7.0 everything works fine, including 2.6.10.