Try to prevent circular import issues

Registering receivers is a tricky business pre-Django 1.7 as they get
registered at the same time when models get registered, but require the
sending model to already be in the app cache. It's surprising we don't
see more issues!

Recommendations:
* Avoid calls to get_model in modules that get called during app init at
  all cost
* Don't use get_model in your own code. Just import directly.
* Switch to Django 1.7 :)

This commit probably fixes the issue in #1205. But get_model calls to
fetch the category, product image and stock record will all fail,
because receivers listen to their signals.

This commit also moves the catalogue receiver into it's own receivers
module, because that's how we do it elsewhere. That won't make a
difference to the issues though.

docs/source/topics/class_loading_explained.rst

@@ -64,6 +64,10 @@ use ``get_class`` when importing classes from Oscar. This means that if someday
 the class is overridden, it will not require code changes. Care should be taken
 when doing this, as this is a tricky trade-off between maintainability and
 added complexity.
+Please note that we cannot recommend ever using ``get_model`` in your own code.
+Especially pre-Django 1.7, model initialisation is a tricky process and it's
+easy to run into circular import issues.
+
 
 Testing
 -------

oscar/apps/customer/history.py

@@ -6,7 +6,6 @@ from django.dispatch import receiver
 
 from oscar.core.loading import get_class
 
-Product = get_model('catalogue', 'Product')
 product_viewed = get_class('catalogue.signals', 'product_viewed')
 
 
@@ -16,6 +15,11 @@ def get(request):
     """
     ids = extract(request)
 
+    # Needs to live in local scope because receivers in this module get
+    # registered during model initialisation
+    # TODO Move this back to global scope once Django < 1.7 support is removed
+    Product = get_model('catalogue', 'Product')
+
     # Reordering as the ID order gets messed up in the query
     product_dict = Product.browsable.in_bulk(ids)
     ids.reverse()
@@ -67,15 +71,3 @@ def update(product, request, response):
         json.dumps(updated_ids),
         max_age=settings.OSCAR_RECENTLY_VIEWED_COOKIE_LIFETIME,
         httponly=True)
-
-
-# Receivers
-
-@receiver(product_viewed)
-def receive_product_view(sender, product, user, request, response, **kwargs):
-    """
-    Receiver to handle viewing single product pages
-
-    Requires the request and response objects due to dependence on cookies
-    """
-    return update(product, request, response)

oscar/apps/customer/models.py

@@ -26,5 +26,5 @@ if not is_model_registered('customer', 'ProductAlert'):
 
 
 if django.VERSION < (1, 7):
-    from oscar.apps.customer.history import *  # noqa
+    from .receivers import *  # noqa
     from .alerts import receivers  # noqa

oscar/apps/customer/receivers.py

@@ -0,0 +1,17 @@
+from django.dispatch import receiver
+
+from oscar.core.loading import get_class
+
+from . import history
+
+product_viewed = get_class('catalogue.signals', 'product_viewed')
+
+
+@receiver(product_viewed)
+def receive_product_view(sender, product, user, request, response, **kwargs):
+    """
+    Receiver to handle viewing single product pages
+
+    Requires the request and response objects due to dependence on cookies
+    """
+    return history.update(product, request, response)

oscar/core/loading.py

@@ -278,6 +278,15 @@ if django.VERSION < (1, 7):
         This is merely a thin wrapper around Django's get_model function.
         Raises LookupError if model isn't found.
         """
+
+        # The snippet below is not useful in production, but helpful to
+        # investigate circular import issues
+        # from django.db.models.loading import app_cache_ready
+        # if not app_cache_ready():
+        #     print(
+        #         "%s.%s accessed before app cache is fully populated!" %
+        #         (app_label, model_name))
+
         model = django_get_model(app_label, model_name, *args, **kwargs)
         if model is None:
             raise LookupError(