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POdjango-rest-framework - trying to set required=False flag on nested 1-to-M?
 

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  1. POdjango-rest-framework - trying to set required=False flag on nested 1-to-M?
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    <p>I'm having some issue with django-rest-framework, and nested objects.</p> <p>I have a Cart object, as well as CartItem, which links back to a Cart:</p> <pre><code>class Cart(models.Model): customer = models.ForeignKey(Customer) date_created = models.DateTimeField(auto_now_add=True) date_modified = models.DateTimeField(auto_now=True) class CartItem(models.Model): cart = models.ForeignKey(Cart, related_name='cartitems') product = models.ForeignKey(Product, help_text='Product in a cart') quantity = models.PositiveIntegerField(default=1, help_text='Quantity of this product.') date_added = models.DateTimeField(auto_now_add=True, help_text='Date that this product was added to the cart.') </code></pre> <p>I've created serializers for both:</p> <pre><code>class CartItemSerializer(serializers.ModelSerializer): product = serializers.HyperlinkedRelatedField(view_name='product-detail') class Meta: model = CartItem class CartSerializer(serializers.ModelSerializer): customer = serializers.HyperlinkedRelatedField(view_name='customer-detail') cartitems = CartItemSerializer(required=False) total_price = serializers.CharField(source='total_price', read_only=True) shipping_cost = serializers.CharField(source='shipping_cost', read_only=True) class Meta: model = Cart fields = ('id', 'customer', 'date_created', 'date_modified', 'cartitems', 'total_price', 'shipping_cost') </code></pre> <p>However, whenever I try to POST to create a new cart, I get an error, assumedly when it tries to set the non-existent CartItem:</p> <pre><code>TypeError at /api/v1/carts/ add() argument after * must be a sequence, not NoneType </code></pre> <p>However, a Cart isn't required to actually have CartItems.</p> <p>Is there any way to get DRF to respect the <code>required=False</code> flag I get on Cart.cartitems?</p> <p>Cheers, Victor</p> <p>EDIT:</p> <p>I took a stab at tracing it through again:</p> <p>It's calling BaseSerializer.save() in rest_framework/serializers.py with a CartSerializer object.</p> <pre><code>def save(self, **kwargs): """ Save the deserialized object and return it. """ if isinstance(self.object, list): [self.save_object(item, **kwargs) for item in self.object] if self.object._deleted: [self.delete_object(item) for item in self.object._deleted] else: self.save_object(self.object, **kwargs) return self.object </code></pre> <p>It then calls save_object() on the same class:</p> <pre><code>def save_object(self, obj, **kwargs): """ Save the deserialized object and return it. """ if getattr(obj, '_nested_forward_relations', None): # Nested relationships need to be saved before we can save the # parent instance. for field_name, sub_object in obj._nested_forward_relations.items(): if sub_object: self.save_object(sub_object) setattr(obj, field_name, sub_object) obj.save(**kwargs) if getattr(obj, '_m2m_data', None): for accessor_name, object_list in obj._m2m_data.items(): setattr(obj, accessor_name, object_list) del(obj._m2m_data) if getattr(obj, '_related_data', None): for accessor_name, related in obj._related_data.items(): if isinstance(related, RelationsList): # Nested reverse fk relationship for related_item in related: fk_field = obj._meta.get_field_by_name(accessor_name)[0].field.name setattr(related_item, fk_field, obj) self.save_object(related_item) # Delete any removed objects if related._deleted: [self.delete_object(item) for item in related._deleted] elif isinstance(related, models.Model): # Nested reverse one-one relationship fk_field = obj._meta.get_field_by_name(accessor_name)[0].field.name setattr(related, fk_field, obj) self.save_object(related) else: # Reverse FK or reverse one-one setattr(obj, accessor_name, related) del(obj._related_data) </code></pre> <p>The Cart object has a <code>_related_data</code> field that is set to a dict:</p> <pre><code>{'cartitems': None} </code></pre> <p>Hence, on the second-last line, it calls setattr in django/db/models/fields/related.py:</p> <pre><code>def __set__(self, instance, value): if instance is None: raise AttributeError("Manager must be accessed via instance") manager = self.__get__(instance) # If the foreign key can support nulls, then completely clear the related set. # Otherwise, just move the named objects into the set. if self.related.field.null: manager.clear() manager.add(*value) </code></pre> <p>It's this last liner (manager.add(*value)) that causes the:</p> <pre><code>TypeError: add() argument after * must be a sequence, not NoneType </code></pre>
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