Support fetching specific db column in OVO

This commit introduces a new method 'get_values' in
neutron_lib.db.model_query module. Callers can leverage
this method to fetch specific field of a OVO.

Related patch in Neutron: https://review.openstack.org/#/c/610182/

Change-Id: I524c7101fa2dcbc5bd78587bf1fc8107faca70b5

neutron_lib/db/model_query.py

@@ -19,8 +19,10 @@ from oslo_db.sqlalchemy import utils as sa_utils
 from sqlalchemy import sql, or_, and_
 from sqlalchemy.ext import associationproxy
 
+from neutron_lib._i18n import _
 from neutron_lib.api import attributes
 from neutron_lib.db import utils as db_utils
+from neutron_lib import exceptions as n_exc
 from neutron_lib.objects import utils as obj_utils
 from neutron_lib.utils import helpers
 
@@ -95,14 +97,23 @@ def get_hooks(model):
     return _model_query_hooks.get(model, {}).values()
 
 
-def query_with_hooks(context, model):
+def query_with_hooks(context, model, field=None):
     """Query with hooks using the said context and model.
 
     :param context: The context to use for the DB session.
     :param model: The model to query.
+    :param field: The column.
     :returns: The query with hooks applied to it.
     """
-    query = context.session.query(model)
+    if field:
+        if hasattr(model, field):
+            field = getattr(model, field)
+        else:
+            msg = _("'%s' is not supported as field") % field
+            raise n_exc.InvalidInput(error_message=msg)
+        query = context.session.query(field)
+    else:
+        query = context.session.query(model)
     # define basic filter condition for model query
     query_filter = None
     if db_utils.model_query_scope_is_project(context, model):
@@ -301,6 +312,12 @@ def get_collection(context, model, dict_func,
     return items
 
 
+def get_values(context, model, field, filters=None):
+    query = query_with_hooks(context, model, field=field)
+    query = apply_filters(query, model, filters, context)
+    return [c[0] for c in query]
+
+
 def get_collection_count(context, model, filters=None):
     """Get the count for a specific collection.
 

neutron_lib/tests/unit/db/test_model_query.py

@@ -56,3 +56,15 @@ class TestHooks(_base.BaseTestCase):
                     self.assertEqual(hook_ref, d.get(k))
                 else:
                     self.assertEqual({}, d.get(k))
+
+    def test_get_values(self):
+        mock_model = mock.Mock()
+        mock_context = mock.Mock()
+        with mock.patch.object(
+                model_query, 'query_with_hooks') as query_with_hooks:
+            query_with_hooks.return_value = [['value1'], ['value2']]
+            values = model_query.get_values(mock_context, mock_model,
+                                            'fake_field')
+        self.assertEqual(['value1', 'value2'], values)
+        query_with_hooks.assert_called_with(
+            mock_context, mock_model, field='fake_field')

releasenotes/notes/add-support-for-fetching-specific-column-in-OVO-81b764b203849776.yaml

@@ -0,0 +1,9 @@
+---
+features:
+  - |
+    Add method ``get_values`` in ``neutron_lib.db.model_query``.
+    This method allow callers to fetch specific column from
+    a database model.
+    Add keyword parameter ``field`` to method ``query_with_hooks``.
+    The default value of this parameter is None.
+    Callers can set this parameter to query specific column.