This feature is actually provided by type guessers (especially the Doctrine ORM one). It does not only work for the length attribute, but for many other usages. So I don't think this particular note should be added.
However, it only works when not specifying any type for the given field in the form, but let the symfony registered type guessers do the work for you.

It's more or less explained in Creating a custom Type Guesser.

I'm not sure that we're talking about the same thing.

When I create a form like here: http://symfony.com/doc/current/book/forms.html#creating-form-classes
...and in the Entity I set the length to 10, I get the maxlength attribute in HTML. However, when I do submit more than 10 characters, I don't get any error message from the validator. This is only the case when I add a rule in validation.yml too.

The DoctrineOrmTypeGuesser, is this enabled by default? If no, where do I have to enable it?

@ThomasLandauer : I confirm we're talking about the same thing. Types guessers are not about validation, they guess the form type to use and some attributes like required or other html attributes.
Type guessers shipped with Symfony are based on Symfony's Validator & Doctrine metadata.

See:

• DoctrineORMTypeGuesser
• ValidatorTypeGuesser

Input type, required, max length & pattern html attributes are guessed thanks to them.

The DoctrineOrmTypeGuesser is enabled by default.
But as I said, type guessers are only active for fields for which you didn't specified explicitely a type when creating the form.

OK, my point is: Being a smart framework, I (personally) would have expected that Symfony automatically uses my Doctrine metadata for validation (i.e. true server-side validation). Since this is not the case, I think it should be explained somewhere that the user is expected to repeat the ORM rules in validation.yml.

I think the entire Forms chapter should be reorganised, prioritizing more on real use-cases (like this one, namely binding a form to a database entity) and less on exotic features like forms with multiple submit buttons. In other words: Before you can tell people about validation groups, you should tell them how validation works at all :-)

I don't insist on my particular wording, neither on this very location. But, again, since binding a form to a database entity is a standard use-case, it should be explained somewhere how validation works in this case.

Hey guys!

Catching up on some old PR's :). I definitely agree with you @ThomasLandauer, and when I went through the current docs (which have been restructured already in many of the ways you suggested), I found out that I think we now have this covered: the forms chapter talks about field option guessing, what it does, and how it works: http://symfony.com/doc/current/forms.html#field-type-options-guessing. And, this conversation comes after talking about real, server-side validation.

So, I'm going to close this PR as finished - please feel free to open a PR if you think we could clarify further.

Thanks!

Um, now the entire "Forms and Doctrine" subsection (https://github.com/ThomasLandauer/symfony-docs/blob/dbe120598ca6fd38b38adff456a2247d3849645b/book/forms.rst#forms-and-doctrine) of the forms chapter is gone!
Was this moved to a separate article?? Can't find it anywhere.....

So while being better in general, the new version of the chapter is even worse than the old one, regarding the interplay with Doctrine. Which is the most widely used use-case of a form, after all.