Remove SubfieldBase metaclass

openedx/core/djangoapps/content/course_overviews/models.py

@@ -551,10 +551,7 @@ class CourseOverview(TimeStampedModel):
         """
         Returns all course keys from course overviews.
         """
-        return [
-            CourseKey.from_string(course_overview['id'])
-            for course_overview in CourseOverview.objects.values('id')
-        ]
+        return CourseOverview.objects.values_list('id', flat=True)
 
     def is_discussion_tab_enabled(self):
         """

openedx/core/djangoapps/util/model_utils.py

@@ -0,0 +1,27 @@
+class Creator(object):
+    """
+    A placeholder class that provides a way to set the attribute on the model.
+    """
+    def __init__(self, field):
+        self.field = field
+
+    def __get__(self, obj, type=None):
+        if obj is None:
+            return self
+        return obj.__dict__[self.field.name]
+
+    def __set__(self, obj, value):
+        obj.__dict__[self.field.name] = self.field.to_python(value)
+
+
+class CreatorMixin(object):
+    """
+    Mixin class to provide SubfieldBase functionality to django fields.
+    See: https://docs.djangoproject.com/en/1.11/releases/1.8/#subfieldbase
+    """
+    def contribute_to_class(self, cls, name, *args, **kwargs):
+        super(CreatorMixin, self).contribute_to_class(cls, name, *args, **kwargs)
+        setattr(cls, name, Creator(self))
+
+    def from_db_value(self, value, expression, connection, context):
+        return self.to_python(value)

openedx/core/djangoapps/xmodule_django/models.py

@@ -7,7 +7,7 @@ import warnings
 from django.core.exceptions import ValidationError
 from django.db import models
 from opaque_keys.edx.keys import BlockTypeKey, CourseKey, UsageKey
-
+from openedx.core.djangoapps.util.model_utils import CreatorMixin
 from xmodule.modulestore.django import modulestore
 
 log = logging.getLogger(__name__)
@@ -68,7 +68,7 @@ def _strip_value(value, lookup='exact'):
     return stripped_value
 
 
-class OpaqueKeyField(models.CharField):
+class OpaqueKeyField(CreatorMixin, models.CharField):
     """
     A django field for storing OpaqueKeys.
 
@@ -81,8 +81,6 @@ class OpaqueKeyField(models.CharField):
     """
     description = "An OpaqueKey object, saved to the DB in the form of a string."
 
-    __metaclass__ = models.SubfieldBase
-
     Empty = object()
     KEY_CLASS = None