Change RawDatastore to do less reflection.

go/src/infra/gae/libs/meta/eg.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 meta
 
 import (
         "golang.org/x/net/context"
 
         "github.com/luci/luci-go/common/errors"
 
         "infra/gae/libs/gae"
+        "infra/gae/libs/gae/helper"
 )
 
 var mark = errors.MakeMarkFn("eg")
 
 // EntityGroupMeta is the model corresponding to the __entity_group__ model in
 // appengine. You shouldn't need to use this struct directly, but instead should
 // use GetEntityGroupVersion.
 type EntityGroupMeta struct {
         Version int64 `gae:"__version__"`
 }
 
 // GetEntityGroupVersion returns the entity group version for the entity group
 // containing root. If the entity group doesn't exist, this function will return
 // zero and a nil error.
 func GetEntityGroupVersion(c context.Context, root gae.DSKey) (int64, error) {
         for root.Parent() != nil {
                 root = root.Parent()
         }
+        rds := gae.GetRDS(c)
         egm := &EntityGroupMeta{}
-        rds := gae.GetRDS(c)
-        err := rds.Get(rds.NewKey("__entity_group__", "", 1, root), egm)
-        if err != gae.ErrDSNoSuchEntity {
-                err = mark(err)
-        } else {
+        err := rds.Get(rds.NewKey("__entity_group__", "", 1, root), helper.GetPLS(egm))
+        ret := egm.Version
+        if err == gae.ErrDSNoSuchEntity {
                 // this is OK for callers. The version of the entity group is effectively 0
                 // in this case.
                 err = nil
         }
-        return egm.Version, err
+        return ret, err
 }