existing_user_fields: Fetch User model only when called

Otherwise it may cause Django start up pains, and does cause migrations
to fail. Thanks to Jonathan Moss for reporting.

oscar/core/compat.py

@@ -37,18 +37,24 @@ def get_user_model():
     raise ImproperlyConfigured("AUTH_USER_MODEL must be of the form"
                                " 'app_label.model_name'")
 
-_user_fields = get_user_model()._meta.fields
-VALID_USER_FORM_FIELD_NAMES = set([field.name for field in _user_fields])
-
 
 def existing_user_fields(fields):
     """
     Starting with Django 1.6, the User model can be overridden  and it is no
     longer safe to assume the User model has certain fields. This helper
     function assists in writing portable forms Meta.fields definitions
     when those contain fields on the User model
+
+    Usage:
+    class UserForm(forms.Form):
+        ...
+        class Meta:
+            # won't break if first_name is not defined on User model
+            fields = existing_user_fields(['first_name', 'last_name'])
     """
-    return list(set(fields) & VALID_USER_FORM_FIELD_NAMES)
+    user_fields = get_user_model()._meta.fields
+    user_field_names = [field.name for field in user_fields]
+    return list(set(fields) & set(user_field_names))
 
 #
 # Python3 compatibility layer

tests/integration/customer/test_custom_user_model.py

@@ -1,21 +1,21 @@
 from django.test import TestCase
 from oscar.apps.customer.forms import ProfileForm
-from oscar.core import compat
+from oscar.core.compat import get_user_model, existing_user_fields
 
 
-class TestCustomUserModel(TestCase):
+class TestACustomUserModel(TestCase):
 
     def setUp(self):
-        self.user_klass = compat.get_user_model()
+        self.user_klass = get_user_model()
 
     def test_can_be_created_without_error(self):
         try:
             self.user_klass.objects.create_user('_', 'a@a.com', 'pa55w0rd')
         except Exception, e:
             self.fail("Unable to create user model: %s" % e)
 
-    def test_extra_field_is_picked_up(self):
-        self.assertTrue('extra_field' in compat.VALID_USER_FORM_FIELD_NAMES)
+    def test_extra_field_is_accessible(self):
+        self.assertTrue('extra_field' in existing_user_fields(['extra_field']))
         self.assertTrue(hasattr(self.user_klass(), 'extra_field'))
 
     def test_profile_form_doesnt_expose_extra_field(self):