Add ability to override just the metadata in a struct.

service/datastore/pls.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 (
         "reflect"
 )
 
@@ skipping 68 matching lines @@
 //   struct Comment {
 //     ID int64 `gae:"$id"`
 //     // "kind" is automatically implied by the struct name: "Comment"
 //
 //     // Parent will be enforced by the application to be a User key.
 //     Parent Key `gae:"$parent"`
 //
 //     // 'Lines' will serialized to the datastore in the field 'Lines'
 //     Lines []string
 //   }
+//
+// A pointer-to-struct may also implement MetaGetterSetter to provide more
+// sophistocated metadata values. Explicitly defined fields (as shown above)
+// always take precedence over fields manipulated by the MetaGetterSetter
+// methods. So if your GetMeta handles "kind", but you explicitly have a
+// $kind field, the $kind field will take precedence and your GetMeta
+// implementation will not be called for "kind".
 func GetPLS(obj interface{}) PropertyLoadSaver {
         v := reflect.ValueOf(obj)
         if v.Kind() != reflect.Ptr || v.Elem().Kind() != reflect.Struct {
                 return &structPLS{c: &structCodec{problem: ErrInvalidEntityType}}
         }
         if v.IsNil() {
                 return &structPLS{c: &structCodec{problem: ErrInvalidEntityType}}
         }
         v = v.Elem()
         c := getCodec(v.Type())
         return &structPLS{v, c}
 }
 
 func getCodec(structType reflect.Type) *structCodec {
         structCodecsMutex.RLock()
         c, ok := structCodecs[structType]
         structCodecsMutex.RUnlock()
         if ok {
                 return c
         }
 
         structCodecsMutex.Lock()
         defer structCodecsMutex.Unlock()
         return getStructCodecLocked(structType)
 }