title: Laravel Many To Many relation returns null
body: <p>I have a class User with many to many relation with itself</p>
  
  <pre><code>public function following(){//Many To Many relationship
      return $this-&gt;belongsToMany(User::class, 'users_users', 'username', 'follower');
  }
  public function followers(){//Many To Many relationship
      return $this-&gt;belongsToMany(User::class, 'users_users', 'follower', 'username');
  }
  </code></pre>
  
  <p>I made the table migration users_users:</p>
  
  <pre><code>public function up()
  {
      Schema::create('users_users', function (Blueprint $table) {
          $table-&gt;increments('id');
          $table-&gt;timestamps();
  
          $table-&gt;string('username');
          $table-&gt;foreign('username')-&gt;references('username')-&gt;on('users');
  
          $table-&gt;string('follower');
          $table-&gt;foreign('follower')-&gt;references('username')-&gt;on('users');
      });
  }
  </code></pre>
  
  <p>and when I try to get the followers of one user, it returns me an empty array when the table is filled with relations.</p>
  
  <pre><code>public function getFollowers(String $username){
      $user = User::where('username',$username)-&gt;get();
      if(!$user){//si no ha encontrado el user con ese id
          return response()-&gt;json(['message' =&gt; 'User not found'],404);
      }
      return response()-&gt;json(['followers' =&gt; $user[0]-&gt;followers],200);
  }
  </code></pre>
  
  <p>Any idea? :/</p>