Update session examples in docs (#8795)

The session examples contained approaches that aren't favorable for an application
and needed an overhaul using either scalar values or an DTO instead of the whole
entity with detach() and merge().

docs/en/cookbook/entities-in-session.rst

@@ -3,42 +3,69 @@ Entities in the Session
 
 There are several use-cases to save entities in the session, for example:
 
-1.  User object
+1.  User data
 2.  Multi-step forms
 
 To achieve this with Doctrine you have to pay attention to some details to get
 this working.
 
-Merging entity into an EntityManager
-------------------------------------
+Updating an entity
+------------------
 
 In Doctrine an entity objects has to be "managed" by an EntityManager to be
-updateable. Entities saved into the session are not managed in the next request
-anymore. This means that you have to register these entities with an
-EntityManager again if you want to change them or use them as part of
-references between other entities. You can achieve this by calling
-``EntityManager#merge()``.
+updatable. Entities saved into the session are not managed in the next request
+anymore. This means that you have to update the entities with the stored session
+data after you fetch the entities from the EntityManager again.
 
-For a representative User object the code to get turn an instance from
-the session into a managed Doctrine object looks like this:
+For a representative User object the code to get data from the session into a
+managed Doctrine object can look like these examples:
+
+Working with scalars
+~~~~~~~~~~~~~~~~~~~~
+
+In simpler applications there is no need to work with objects in sessions and you can use
+separate session elements.
 
 .. code-block:: php
 
     <?php
     require_once 'bootstrap.php';
-    $em = GetEntityManager(); // creates an EntityManager 
 
     session_start();
-    if (isset($_SESSION['user']) && $_SESSION['user'] instanceof User) {
-        $user = $_SESSION['user']; 
-        $user = $em->merge($user);
+    if (isset($_SESSION['userId']) && is_int($_SESSION['userId'])) {
+        $userId = $_SESSION['userId'];
+
+        $em = GetEntityManager(); // creates an EntityManager
+        $user = $em->find(User::class, $userId);
+
+        $user->setValue($_SESSION['storedValue']);
+
+        $em->flush();
     }
 
-.. note::
+Working with custom data transfer objects
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+If objects are needed, we discourage the storage of entity objects in the session. It's
+preferable to use a `DTO (data transfer object) <https://en.wikipedia.org/wiki/Data_transfer_object>`_
+instead and merge the DTO data later with the entity.
+
+.. code-block:: php
+
+    <?php
+    require_once 'bootstrap.php';
+
+    session_start();
+    if (isset($_SESSION['user']) && $_SESSION['user'] instanceof UserDto) {
+        $userDto = $_SESSION['user'];
 
-    A frequent mistake is not to get the merged user object from the return
-    value of ``EntityManager#merge()``. The entity object passed to merge is
-    not necessarily the same object that is returned from the method.
+        $em = GetEntityManager(); // creates an EntityManager
+        $userEntity = $em->find(User::class, $userDto->getId());
+
+        $userEntity->populateFromDto($userDto);
+
+        $em->flush();
+    }
 
 Serializing entity into the session
 -----------------------------------
@@ -47,22 +74,20 @@ Entities that are serialized into the session normally contain references to
 other entities as well. Think of the user entity has a reference to their
 articles, groups, photos or many other different entities. If you serialize
 this object into the session then you don't want to serialize the related
-entities as well. This is why you should call ``EntityManager#detach()`` on this
-object or implement the __sleep() magic method on your entity.
+entities as well. This is why you shouldn't serialize an entity and use
+only the needed values of it. This can happen with the help of a DTO.
 
 .. code-block:: php
 
     <?php
     require_once 'bootstrap.php';
+
     $em = GetEntityManager(); // creates an EntityManager 
 
     $user = $em->find("User", 1);
-    $em->detach($user);
-    $_SESSION['user'] = $user;
+    $userDto = new UserDto($user->getId(), $user->getFirstName(), $user->getLastName());
+    // or "UserDto::createFrom($user);", but don't store an entity in a property. Only its values without relations.
 
-.. note::
+    $_SESSION['user'] = $userDto;
 
-    When you called detach on your objects they get "unmanaged" with that
-    entity manager. This means you cannot use them as part of write operations
-    during ``EntityManager#flush()`` anymore in this request.