minor symfony#10068 Add necessary call to Form::isSubmitted and remove boilerplate (keichinger, javiereguiluz)

This PR was submitted for the master branch but it was merged into the 4.0 branch instead (closes symfony#10068).

Discussion
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Add necessary call to Form::isSubmitted and remove boilerplate

This code example was the last parts of the docs that wasn't calling `Form::isSubmitted()` as required as of Symfony 4.0.

Other than that I've simplified the action a little bit to remove some rather unnecessary noise in an already little more complicated example to ease reading and understanding it. If you want to keep the boilerplate, I'm more than happy to remove it from this PR :)

Commits
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ae265d7 Update form_collections.rst
f140342 Minor refactor
6f2c417 Add necessary call to Form::isSubmitted and remove boilerplate

Author: javiereguiluz

form/form_collections.rst

@@ -655,12 +655,9 @@ the relationship between the removed ``Tag`` and ``Task`` object.
         use Doctrine\Common\Collections\ArrayCollection;
 
         // ...
-        public function edit($id, Request $request)
+        public function edit($id, Request $request, EntityManagerInterface $entityManager)
         {
-            $entityManager = $this->getDoctrine()->getManager();
-            $task = $entityManager->getRepository(Task::class)->find($id);
-
-            if (!$task) {
+            if (null === $task = $entityManager->getRepository(Task::class)->find($id)) {
                 throw $this->createNotFoundException('No task found for id '.$id);
             }
 
@@ -675,8 +672,7 @@ the relationship between the removed ``Tag`` and ``Task`` object.
 
             $editForm->handleRequest($request);
 
-            if ($editForm->isValid()) {
-
+            if ($editForm->isSubmitted() && $editForm->isValid()) {
                 // remove the relationship between the tag and the Task
                 foreach ($originalTags as $tag) {
                     if (false === $task->getTags()->contains($tag)) {