Sort CellMappingList.get_all() for safety

The result of this method should be stable in database order, but since
we depend on it for some pagination, this patch sorts the results by
the most efficient thing we have to be sure.

Related to blueprint cells-aware-api

Change-Id: I81fb8a2a342ad54adc386c02a5fe4a90800fcbb0

nova/objects/cell_mapping.py

@@ -10,6 +10,8 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+from sqlalchemy.sql.expression import asc
+
 from nova.db.sqlalchemy import api as db_api
 from nova.db.sqlalchemy import api_models
 from nova import exception
@@ -127,7 +129,8 @@ class CellMappingList(base.ObjectListBase, base.NovaObject):
     @staticmethod
     @db_api.api_context_manager.reader
     def _get_all_from_db(context):
-        return context.session.query(api_models.CellMapping).all()
+        return context.session.query(api_models.CellMapping).order_by(
+            asc(api_models.CellMapping.id)).all()
 
     @base.remotable_classmethod
     def get_all(cls, context):

nova/tests/unit/objects/test_cell_mapping.py

@@ -138,6 +138,20 @@ class _TestCellMappingListObject(object):
         get_all_from_db.assert_called_once_with(self.context)
         self.compare_obj(mapping_obj.objects[0], db_mapping)
 
+    def test_get_all_sorted(self):
+        for ident in (10, 3):
+            cm = objects.CellMapping(context=self.context,
+                                     id=ident,
+                                     uuid=getattr(uuids, 'cell%i' % ident),
+                                     transport_url='fake://%i' % ident,
+                                     database_connection='fake://%i' % ident)
+            cm.create()
+        obj = objects.CellMappingList.get_all(self.context)
+        ids = [c.id for c in obj]
+        # Find the two normal cells, plus the two we created, but in the right
+        # order
+        self.assertEqual([1, 2, 3, 10], ids)
+
 
 class TestCellMappingListObject(test_objects._LocalTest,
                                 _TestCellMappingListObject):