Make model classes inherit their parents fields attribute

common/lib/xmodule/xmodule/model.py

@@ -121,7 +121,12 @@ class ModelMetaclass(type):
                 v._name = n
                 fields.append(v)
         fields.sort()
-        attrs['fields'] = fields
+        attrs['fields'] = sum([
+            base.fields
+            for base
+            in bases
+            if hasattr(base, 'fields')
+        ], fields)
 
         return super(ModelMetaclass, cls).__new__(cls, name, bases, attrs)
 

common/lib/xmodule/xmodule/tests/test_model.py

@@ -15,12 +15,21 @@ def test_model_metaclass():
         def __init__(self, model_data):
             self._model_data = model_data
 
+    class ChildClass(ModelMetaclassTester):
+        pass
+
     assert hasattr(ModelMetaclassTester, 'field_a')
     assert hasattr(ModelMetaclassTester, 'field_b')
 
     assert_in(ModelMetaclassTester.field_a, ModelMetaclassTester.fields)
     assert_in(ModelMetaclassTester.field_b, ModelMetaclassTester.fields)
 
+    assert hasattr(ChildClass, 'field_a')
+    assert hasattr(ChildClass, 'field_b')
+
+    assert_in(ChildClass.field_a, ChildClass.fields)
+    assert_in(ChildClass.field_b, ChildClass.fields)
+
 
 def test_parent_metaclass():