Merged in 3 more commits of Jon's updating products

oscar/product/admin.py

 from product.models import Type
 from product.models import Item
 from product.models import Attribute
+from product.models import AttributeTypeMembership
 from django.contrib import admin
 
 class AttributeInline(admin.TabularInline):
 
 
 admin.site.register(AttributeType)
+admin.site.register(AttributeTypeMembership)
 admin.site.register(Type)
 admin.site.register(Item, ItemAdmin)
 admin.site.register(Attribute)

oscar/product/models.py

 from django.db import models
 
 class AttributeType(models.Model):
-    """Defines a product atrribute type"""
+    """Defines a product attribute type"""
     name = models.CharField(max_length = 128)
 
     def __unicode__(self):
 class Type(models.Model):
     """Defines a product type"""
     name = models.CharField(max_length = 128)
-    attribute_types = models.ManyToManyField('product.AttributeType')
+    attribute_types = models.ManyToManyField('product.AttributeType', through = 'product.AttributeTypeMembership')
 
     def __unicode__(self):
         return self.name
 
+class AttributeTypeMembership(models.Model):
+    RELATIONSHIP_CHOICES = (
+        ('optional', 'optional'),
+        ('required', 'required'),
+        ('required_basket', 'required for purchase'),
+    )
+    type = models.ForeignKey('product.Type')
+    attribute_type = models.ForeignKey('product.AttributeType')
+    relation_type = models.CharField(max_length = 16, choices = RELATIONSHIP_CHOICES, default = 'optional')
+    
+    def __unicode__(self):
+        return "%s -> %s (%s)" % (self.type.name, self.attribute_type.name, self.relation_type)
+    
+
 class Item(models.Model):
     """The base product object"""
     name = models.CharField(max_length = 128)
             A boolean if the product is valid
         """
         required_attribute_names = []
-        for attribute_type in self.type.attribute_types.all():
+        for attribute_type in self.type.attribute_types.filter(attributetypemembership__relation_type = 'required'):
             required_attribute_names.append(attribute_type.name)
 
         for attribute in self.attribute_set.all():
-            required_attribute_names.remove(attribute.attribute_type.name)
+            if attribute.attribute_type.name in required_attribute_names:
+                required_attribute_names.remove(attribute.attribute_type.name)
 
         return 0 == len(required_attribute_names)