Allow unassigning categories from products

This has been impossible so far. It is easily achieved by setting
can_delete=True on the formset.

I spent some time trying to get the nice solution working, which would
be that assigning an empty category means deleting that ProductCategory
instance. But I haven't found a good solution that avoids problems like
in #1267. I have since come to the conclusion that enabling can_delete
is the right thing to do. One can then explicitly hide the checkbox,
but set the correct hidden field in Javascript on the client side if
an empty value is assigned.

The fields section has been moved from the factory declaration to the
ProductForm Meta class to allow for easier customisation.

Fixes #1289

docs/source/releases/v0.7.rst

@@ -173,7 +173,10 @@ Bugfixes
 * Suspended and consumed offers are no longer returned by the "active" offer
   manager. (`#1228`_).
 
+* Products can now be removed from categories (`#1289`_).
+
 .. _`#1228`: https://github.com/tangentlabs/django-oscar/issues/1228
+.. _`#1289`: https://github.com/tangentlabs/django-oscar/issues/1289
 .. _`be04d46639`: https://github.com/tangentlabs/django-oscar/commit/
 
 Backwards incompatible changes in 0.7

oscar/apps/dashboard/catalogue/forms.py

@@ -387,16 +387,18 @@ class ProductCategoryForm(forms.ModelForm):
 
     class Meta:
         model = ProductCategory
+        fields = ('category', )
 
 
 BaseProductCategoryFormSet = inlineformset_factory(
-    Product, ProductCategory, form=ProductCategoryForm,
-    fields=('category',), extra=1, can_delete=False)
+    Product, ProductCategory, form=ProductCategoryForm, extra=1,
+    can_delete=True)
 
 
 class ProductCategoryFormSet(BaseProductCategoryFormSet):
 
     def __init__(self, product_class, user, *args, **kwargs):
+        # This function just exists to drop the extra arguments
         super(ProductCategoryFormSet, self).__init__(*args, **kwargs)
 
     def clean(self):
@@ -419,6 +421,7 @@ class ProductCategoryFormSet(BaseProductCategoryFormSet):
 
 
 class ProductImageForm(forms.ModelForm):
+
     class Meta:
         model = ProductImage
         exclude = ('display_order',)