title: AttributeError with Django REST Framework and a ManyToMany relationship
body: <p>Trying to access my json page I get this error!</p>
  
  <pre><code>AttributeError at /project/api/1.json
  Got AttributeError when attempting to get a value for field `title` on serializer `TaskSerializer`.
  The serializer field might be named incorrectly and not match any attribute or key on the `RelatedManager` instance.
  Original exception text was: 'RelatedManager' object has no attribute 'title'.
  </code></pre>
  
  <p>I have a Many to Many relationship with my models:</p>
  
  <pre><code>class Project(models.Model):
      owner = models.ForeignKey('auth.User')
      title = models.CharField(max_length=100)
      slug = models.SlugField(unique=True)
      created_date = models.DateTimeField(auto_now_add=True, auto_now=False)
      updated_date = models.DateTimeField(auto_now_add=False, auto_now=True)
  
      def __str__(self):
          return self.title
  
      def save(self, **kwargs):
          super(Project, self, **kwargs).save()
          self.slug = slugify(self.title)
          super(Project, self, **kwargs).save()
  
      def create(self):
          pass
  
  
  class Task(models.Model):
      title = models.CharField(max_length=100)
      description = models.TextField(blank=True)
      completed = models.BooleanField(default=False)
      project = models.ForeignKey('Project', related_name="tasks")
      dependency = models.ManyToManyField('self', through='Dependency', null=True, 
          blank=True, through_fields=('task', 'sub_task'), symmetrical=False)
  
      def sub_tasks(self, **kwargs):
          qs = self.dependency.filter(**kwargs)
          for sub_task in qs:
              qs = qs | sub_task.sub_tasks(**kwargs)
          return qs
  
      def __str__(self):
          return self.title
  
  class Dependency(models.Model):
      task = models.ForeignKey(Task, related_name="dependency_task")
      sub_task = models.ForeignKey(Task, related_name="dependency_sub_task")
  </code></pre>
  
  <p>And these serializers:</p>
  
  <pre><code>class TaskSerializer(serializers.ModelSerializer):
      class Meta:
          model = Task
          fields = ('id', 'title', 'project', 'completed',)
  
  
  class ProjectSerializer(serializers.ModelSerializer):
      tasks = TaskSerializer()
      class Meta:
          model = Project
          fields = ('id', 'title', 'tasks',)
  </code></pre>
  
  <p>How can I get round this? RelatedManager tells me something is disagreeing with my M2M link, but why/how? I couldn't see anything <a href="http://www.django-rest-framework.org/api-guide/relations/#reverse-relations" rel="nofollow noreferrer">here</a> about Attribute Errors.</p>
  
  <p>This question seems related, but setting <code>many=False</code> doesn't do anything.</p>
  
  <p><a href="https://stackoverflow.com/q/17253164/1075247">AttributeError with Django REST Framework and MongoEngine</a></p>