Fix error message from KeyForObj when passing an invalid struct.

service/datastore/raw_interface_test.go

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package datastore
 
 import (
         "testing"
 
         . "github.com/smartystreets/goconvey/convey"
 )
 
 func TestMultiMetaGetter(t *testing.T) {
         t.Parallel()
 
         Convey("Test MultiMetaGetter", t, func() {
                 Convey("nil", func() {
                         mmg := NewMultiMetaGetter(nil)
                         val, err := mmg.GetMeta(7, "hi")
                         So(err, ShouldEqual, ErrMetaFieldUnset)
                         So(val, ShouldBeNil)
 
-                        So(mmg.GetMetaDefault(7, "hi", "value"), ShouldEqual, "value")
+                        v, err := GetMetaDefault(mmg.GetSingle(7), "hi", "value")
+                        So(err, ShouldBeNil)
+                        So(v, ShouldEqual, "value")
 
                         m := mmg.GetSingle(10)
                         val, err = m.GetMeta("hi")
                         So(err, ShouldEqual, ErrMetaFieldUnset)
                         So(val, ShouldBeNil)
-                        So(m.GetMetaDefault("hi", "value"), ShouldEqual, "value")
+
+                        v, err = GetMetaDefault(m, "hi", "value")
+                        So(err, ShouldBeNil)
+                        So(v, ShouldEqual, "value")
                 })
 
                 Convey("stuff", func() {
                         pmaps := []PropertyMap{{}, nil, {}}
                         So(pmaps[0].SetMeta("hi", "thing"), ShouldBeNil)
                         So(pmaps[2].SetMeta("key", 100), ShouldBeNil)
                         mmg := NewMultiMetaGetter(pmaps)
 
                         // oob is OK
-                        So(mmg.GetMetaDefault(7, "hi", "value"), ShouldEqual, "value")
+                        v, err := GetMetaDefault(mmg.GetSingle(7), "hi", "value")
+                        So(err, ShouldBeNil)
+                        So(v, ShouldEqual, "value")
 
                         // nil is OK
-                        So(mmg.GetMetaDefault(1, "key", true), ShouldEqual, true)
+                        v, err = GetMetaDefault(mmg.GetSingle(1), "key", true)
+                        So(err, ShouldBeNil)
+                        So(v, ShouldEqual, true)
 
                         val, err := mmg.GetMeta(0, "hi")
                         So(err, ShouldBeNil)
                         So(val, ShouldEqual, "thing")
 
-                        So(mmg.GetMetaDefault(2, "key", 20), ShouldEqual, 100)
+                        v, err = GetMetaDefault(mmg.GetSingle(2), "key", 20)
+                        So(err, ShouldBeNil)
+                        So(v, ShouldEqual, 100)
                 })
         })
 }