Add 'keys_only' method to Query.

Sets the projection to include only entity keys.

Supercedes #466.

Author: tseaver

gcloud/datastore/query.py

@@ -246,6 +246,10 @@ def projection(self, projection):
             projection = [projection]
         self._projection[:] = projection
 
+    def keys_only(self):
+        """Set the projection to include only keys."""
+        self._projection[:] = ['__key__']
+
     @property
     def order(self):
         """Names of fields used to sort query results.

gcloud/datastore/test_query.py

@@ -250,6 +250,12 @@ def test_projection_setter_multiple_calls(self):
         query.projection = _PROJECTION2
         self.assertEqual(query.projection, _PROJECTION2)
 
+    def test_keys_only(self):
+        _KIND = 'KIND'
+        query = self._makeOne(_KIND)
+        query.keys_only()
+        self.assertEqual(query.projection, ['__key__'])
+
     def test_order_setter_empty(self):
         _KIND = 'KIND'
         query = self._makeOne(_KIND, order=['foo', '-bar'])