title: FOSUserBundle User addGroup does not save in the database
body: <p>I am trying to add users to role groups but for some reason I cannot achieve this.
  I followed the documentation on symfony website <a href="http://symfony.com/doc/current/bundles/FOSUserBundle/groups.html" rel="nofollow">http://symfony.com/doc/current/bundles/FOSUserBundle/groups.html</a>. The object is created and it's correct but on flush nothing happens.</p>
  
  <p>UserRoleGroups are already defined in the DB.</p>
  
  <blockquote>
    <p>saving </p>
  </blockquote>
  
  <pre><code>$oGroup = $this-&gt;em-&gt;getRepository('UserBundle:UserRoleGroups')-&gt;findOneByName($sRole);
  
  $oUser-&gt;addGroup($oGroup);
  $this-&gt;em-&gt;persist($oUser);
  $this-&gt;em-&gt;flush();
  </code></pre>
  
  <p>Any other changes I do to $oUser object are saved except for this one.</p>
  
  <blockquote>
    <p>User Entity</p>
  </blockquote>
  
  <pre><code>class User extends BaseUser
  {
  
      /**
       * @ORM\Id
       * @ORM\Column(type="integer")
       * @ORM\GeneratedValue(strategy="AUTO")
       */
      protected $id;
  
      /**
       * @ORM\ManyToMany(
       *     targetEntity="App\UserBundle\Entity\UserRoleGroups"
       * )
       * @ORM\JoinTable(
       *     name="users_user_role_group",
       *     joinColumns={@ORM\JoinColumn(name="user_id", referencedColumnName="id")},
       *     inverseJoinColumns={@ORM\JoinColumn(name="group_id", referencedColumnName="id")}
       * )
       */
      protected $userRolesGroup;
  </code></pre>
  
  <blockquote>
    <p>UserRoleGroup Entity</p>
  </blockquote>
  
  <pre><code>class UserRoleGroups extends BaseGroup
  {
  
      /**
       * @ORM\Id
       * @ORM\Column(type="integer")
       * @ORM\GeneratedValue(strategy="AUTO")
       */
      protected $id;
  </code></pre>