diff --git a/nova/test.py b/nova/test.py
index caf1f89b8..085629f21 100644
--- a/nova/test.py
+++ b/nova/test.py
@@ -169,7 +169,14 @@ class TestCase(testtools.TestCase):
     Due to the slowness of DB access, please consider deriving from
     `NoDBTestCase` first.
     """
+    # USES_DB is set to False for tests that inherit from NoDBTestCase.
     USES_DB = True
+    # USES_DB_SELF is set to True in tests that specifically want to use the
+    # database but need to configure it themselves, for example to setup the
+    # API DB but not the cell DB. In those cases the test will override
+    # USES_DB_SELF = True but inherit from the NoDBTestCase class so it does
+    # not get the default fixture setup when using a database (which is the
+    # API and cell DBs, and adding the default flavors).
     USES_DB_SELF = False
     REQUIRES_LOCKING = False
 
diff --git a/nova/tests/functional/db/test_aggregate.py b/nova/tests/functional/db/test_aggregate.py
index 74a713e55..5d8810f36 100644
--- a/nova/tests/functional/db/test_aggregate.py
+++ b/nova/tests/functional/db/test_aggregate.py
@@ -21,7 +21,6 @@ from nova.db.sqlalchemy import api as db_api
 from nova.db.sqlalchemy import api_models
 from nova import exception
 from nova import test
-from nova.tests import fixtures
 from nova.tests.unit import matchers
 from nova.tests.unit.objects.test_objects import compare_obj as base_compare
 from nova.tests import uuidsentinel
@@ -110,14 +109,10 @@ def _aggregate_metadata_get_all(context, aggregate_id):
     return metadata
 
 
-class AggregateObjectDbTestCase(test.NoDBTestCase):
-
-    USES_DB_SELF = True
+class AggregateObjectDbTestCase(test.TestCase):
 
     def setUp(self):
         super(AggregateObjectDbTestCase, self).setUp()
-        self.useFixture(fixtures.Database())
-        self.useFixture(fixtures.Database(database='api'))
         self.context = context.RequestContext('fake-user', 'fake-project')
 
     def test_in_api(self):
@@ -495,15 +490,12 @@ def compare_obj(test, result, source):
                         comparators={'updated_at': updated_at_comparator})
 
 
-class AggregateObjectCellTestCase(test.NoDBTestCase):
+class AggregateObjectCellTestCase(test.TestCase):
     """Tests for the case where all aggregate data is in Cell DB"""
-    USES_DB_SELF = True
 
     def setUp(self):
         super(AggregateObjectCellTestCase, self).setUp()
         self.context = context.RequestContext('fake-user', 'fake-project')
-        self.useFixture(fixtures.Database())
-        self.useFixture(fixtures.Database(database='api'))
         self._seed_data()
 
     def _seed_data(self):
@@ -610,13 +602,10 @@ class AggregateObjectMigrationTestCase(AggregateObjectCellTestCase):
         self.assertEqual(new_agg.name, result.name)
 
 
-class AggregateMigrationTestCase(test.NoDBTestCase):
-    USES_DB_SELF = True
+class AggregateMigrationTestCase(test.TestCase):
 
     def setUp(self):
         super(AggregateMigrationTestCase, self).setUp()
-        self.useFixture(fixtures.Database())
-        self.useFixture(fixtures.Database(database='api'))
         self.context = context.get_admin_context()
 
     def test_migration(self):