(Proposal) Added a note on forms validation

best_practices/forms.rst

@@ -137,6 +137,34 @@ view layer:
         <input type="submit" class="btn" value="Create" />
     {{ form_end(form) }}
 
+Validation
+----------
+
+The `constraints`_ option allows you to attach `validation constraints`_ to any
+form field. However, doing that prevents the validation from being reused in
+other forms or other places where the mapped object is used.
+
+.. best-practice::
+
+    Do not define your validation constraints in the form but on the object the
+    form is mapped to.
+
+For example, to validate that the title of the post edited with a form is not
+blank, add the following in the ``Post`` object::
+
+    // src/Entity/Post.php
+
+    // ...
+    use Symfony\Component\Validator\Constraints as Assert;
+
+    class Post
+    {
+        /**
+         * @Assert\NotBlank()
+         */
+        public $title;
+    }
+
 Rendering the Form
 ------------------
 
@@ -190,3 +218,6 @@ submit. Both those actions will be almost identical. So it's much simpler to let
 ``new()`` handle everything.
 
 Next: :doc:`/best_practices/i18n`
+
+.. _`constraints`: https://symfony.com/doc/current/reference/forms/types/form.html#constraints
+.. _`validation constraints`: https://symfony.com/doc/current/reference/constraints.html