The content of this article is no longer relevant because in current Symfony versions, form classes are services by default and you can inject services in them using the :doc:`service autowiring </service_container/autowiring>` feature.
Read the article about :doc:`creating custom form types </form/create_custom_field_type>` to see an example of how to inject the database service into a form type. In the same article you can also read about :ref:`configuration options for form types <form-type-config-options>`, which is another way of passing services to forms.