title: Call to undefined method, ArrayCollection
body: <p>I'm new of Symfony, I made a web-application with it, in witch a User is in relation ManyToMany with an Entity "Mission". It doesn't work, because I get the error</p>
  
  <pre><code> FatalErrorException: Error: Call to undefined method 
   Acme\ManagementBundle\Entity\UserGroundStation::getMission()
  </code></pre>
  
  <p>Te getMission is defined in the MissionEntity, and called in the construct. Do I have to define it again? I don't understand.</p>
  
  <p>My User is:</p>
  
  <pre><code> &lt;?php
  
   namespace Acme\ManagementBundle\Entity;
  
   use Doctrine\ORM\Mapping as ORM;
   use FOS\UserBundle\Model\User as BaseUser;
   use Doctrine\Common\Collections\ArrayCollection;
   use Doctrine\Common\Collections\Collection;
  
  
   /**
    * @ORM\Entity
    * @ORM\HasLifecycleCallbacks()
    * @ORM\Table(name="user")
    * @ORM\InheritanceType("JOINED")
    * @ORM\DiscriminatorColumn(name="type", type="string")
    * @ORM\DiscriminatorMap({"user_one" = "UserGroundStation", "user_two" = "UserOperator"})
    * @ORM\MappedSuperclass()
    *
    */
   abstract class User extends BaseUser
   {
       /**
        * @ORM\Id
        * @ORM\Column(type="integer")
        * @ORM\GeneratedValue(strategy="AUTO")
        */
       protected $id;
           /**
        * @ORM\ManyToMany(targetEntity="Acme\ManagementBundle\Entity\Mission", mappedBy="users")
        */
       protected $missions;
  
       public function __construct(){
           parent::__construct();
           $this -&gt; missions = new ArrayCollection();
       }
  
       public function setMissions(Collection $missions){
           $this -&gt; missions = $missions;
           return $this;
       }
  
       public function addMission($mission){
           $this -&gt; missions -&gt; add($mission);
           return $this;
       }
  
       public function getMissions(){
           return $this -&gt; missions;
       }
  
   }
  </code></pre>
  
  <p>And the Mission Entity is:</p>
  
  <pre><code>  /**
    * @ORM\Entity
    */
   class Mission {
       /** 
        * @ORM\Id
        * @ORM\GeneratedValue
        * @ORM\Column(type="integer")
        * @var integer
        */
       protected $id;
           /** 
        * @ORM\Column(type="string", length=60)
        * @var String
        */
       protected $name;
       /** 
        * @ORM\Column(type="string", length=600)
        * @var String
        */
       protected $description;
       /**
        * @ORM\ManyToMany(targetEntity="Acme\ManagementBundle\Entity\User", inversedBy="users")
        */
       protected $users;
  
       public function __construct(){
           $this -&gt; users = new ArrayCollection();
       }
  
       public function setUsers(Collection $users){
           $this -&gt; users = $users;
           return $this;
       }
  
       public function addUser($user){
           $this -&gt; users -&gt; add($user);
           return $this;
       }
  
       public function getUsers(){
           return $this -&gt; users;
       }
       //setters and getters for id, name and description
   }
  </code></pre>
  
  <p>How to solve this? Thank you</p>