Warning: You are browsing the documentation for version 5.1 which is not maintained anymore.
If some of your projects are still using this version, consider upgrading.
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 service autowiring feature.
Read the article about creating custom form types to see an example of how to inject the database service into a form type. In the same article you can also read about configuration options for form types, which is another way of passing services to forms.