Add meta package to datastore.

service/datastore/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 (
+        "github.com/luci/gae/service/datastore"
+        "golang.org/x/net/context"
+)
+
+// 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         `gae:"$kind,__entity_group__"`
+        id     int64          `gae:"$id,1"`
+        Parent *datastore.Key `gae:"$parent"`
+
+        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, key *datastore.Key) (int64, error) {
+        ds := datastore.Get(c)
+        egm := &EntityGroupMeta{Parent: key.Root()}
+        err := ds.Get(egm)
+        ret := egm.Version
+        if err == datastore.ErrNoSuchEntity {
+                // this is OK for callers. The version of the entity group is effectively 0
+                // in this case.
+                err = nil
+        }
+        return ret, err
+}