title: django-tastypie: Posting to a Resource having ManytoMany field with through relationship
body: <p>I'm working on a API for a project and I have a relationship Order/Products through OrderProducts like this:</p>
  
  <p>In models.py</p>
  
  <pre><code>class Product(models.Model):
      ...
  
  class Order(models.Model):
      products = models.ManyToManyField(Product, verbose_name='Products', through='OrderProducts')
      ...
  
  class OrderProducts(models.Model):
      order = models.ForeignKey(Order)
      product = models.ForeignKey(Product)
      ...
  </code></pre>
  
  <p>Now, when I load an Order through the API I'd like to get the related Products as well, so I tried this (with django-tastypie):</p>
  
  <p>In order/api.py</p>
  
  <pre><code>class OrderResource(ModelResource):
      products = fields.ToManyField('order.api.ProductResource', products, full=True)
  
      class Meta:
          queryset = Order.objects.all()
          resource_name = 'order'
  </code></pre>
  
  <p>Everything works for listing Order resources. I get order resources with product data embedded. </p>
  
  <p>The problem is that I am not able to create or edit Order objects using the api. Since I am using a through model in ManytoMany relation, the ManyToManyField(products) does not have the .add() methods. But tastypie is trying to call .add() on the products field in OrderResource when posting/putting data to it. </p>
  
  <pre><code>{"error_message": "'ManyRelatedManager' object has no attribute 'add'", "traceback": "Traceback (most recent call last):\n\n  File \"/Library/Python/2.7/site-packages/tastypie/resources.py\", line 192, in wrapper\n    response = callback(request, *args, **kwargs)\n\n  File \"/Library/Python/2.7/site-packages/tastypie/resources.py\", line 397, in dispatch_list\n    return self.dispatch('list', request, **kwargs)\n\n  File \"/Library/Python/2.7/site-packages/tastypie/resources.py\", line 427, in dispatch\n    response = method(request, **kwargs)\n\n  File \"/Library/Python/2.7/site-packages/tastypie/resources.py\", line 1165, in post_list\n    updated_bundle = self.obj_create(bundle, request=request, **self.remove_api_resource_names(kwargs))\n\n  File \"/Library/Python/2.7/site-packages/tastypie/resources.py\", line 1784, in obj_create\n    self.save_m2m(m2m_bundle)\n\n  File \"/Library/Python/2.7/site-packages/tastypie/resources.py\", line 1954, in save_m2m\n    related_mngr.add(*related_objs)\n\nAttributeError: 'ManyRelatedManager' object has no attribute 'add'\n"}
  </code></pre>