Reland: Refactor current GAE abstraction library to be free of the SDK*

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"
 
-        "appengine/datastore"
-
         "github.com/luci/luci-go/common/errors"
 
-        "infra/gae/libs/wrapper"
+        "infra/gae/libs/gae"
 )
 
 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 {
-        _kind string `datastore:"-" goon:"kind,__entity_group__"`
-
-        ID     int64          `datastore:"-" goon:"id"`
-        Parent *datastore.Key `datastore:"-" goon:"parent"`
-
-        Version int64 `datastore:"__version__"`
+        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 *datastore.Key) (int64, error) {
+func GetEntityGroupVersion(c context.Context, root gae.DSKey) (int64, error) {
         for root.Parent() != nil {
                 root = root.Parent()
         }
-        egm := &EntityGroupMeta{ID: 1, Parent: root}
-        err := wrapper.GetDS(c).Get(egm)
-        if err != datastore.ErrNoSuchEntity {
+        egm := &EntityGroupMeta{}
+        rds := gae.GetRDS(c)
+        err := rds.Get(rds.NewKey("__entity_group__", "", 1, root), egm)
+        if err != gae.ErrDSNoSuchEntity {
                 err = mark(err)
         } else {
                 // this is OK for callers. The version of the entity group is effectively 0
                 // in this case.
                 err = nil
         }
         return egm.Version, err
 }