title: 'collections.OrderedDict' object has no attribute 'username'
body: <p>I have an "Event" model and it has a <code>many2many</code> field with default user model. </p>
  
  <pre><code>class Event(models.Model):
      name = models.CharField(max_length=50)
      type = models.CharField(max_length=50)
      location = models.CharField(max_length=255)
      start_hour = models.CharField(max_length=50)
      end_hour = models.CharField(max_length=50)
      creator = models.CharField(max_length=50)
      info = models.CharField(max_length=255, default='')
      users = models.ManyToManyField(User)
  
      def __str__(self):
          return self.name
  </code></pre>
  
  <p>Now, I am trying to update this <code>many2many</code> field like following;</p>
  
  <pre><code>//my serializers
  
  class UserSerializer(serializers.ModelSerializer):
      class Meta:
          model = User
          fields = ('id',)
  
  
  class EventSerializer(serializers.ModelSerializer):
      users = UserSerializer(many=True)
  
      class Meta:
          model = Event
          fields = ('id', 'users')
  
      def update(self, instance, validated_data):
          submitted_users = validated_data.get('users')
          if submitted_users:
              for user in submitted_users:
                  user_instance = User.objects.get(id=user.id)
                  instance.users.add(user_instance)
          instance.save()
          return instance
  
  //views.py
  
  class UpdateParticipants(generics.RetrieveUpdateDestroyAPIView):
      queryset = Event.objects.all()
      serializer_class = EventSerializer
  </code></pre>
  
  <p>However, I am getting an error like in the belowe image</p>
  
  <p><a href="https://i.stack.imgur.com/3TMxT.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/3TMxT.png" alt="enter image description here"></a></p>
  
  <p>// this is the APIView that I used</p>
  
  <p><a href="https://i.stack.imgur.com/2hdbq.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/2hdbq.png" alt="enter image description here"></a></p>