Add ability to override just the metadata in a struct.

service/datastore/properties.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 (
         "encoding/base64"
         "errors"
         "fmt"
@@ skipping 662 matching lines @@
         // error occurs, it's returned via error. If non-fatal conversion errors
         // occur, error will be a MultiError containing one or more ErrFieldMismatch
         // objects.
         Load(PropertyMap) error
 
         // Save returns the current property as a PropertyMap. if withMeta is true,
         // then the PropertyMap contains all the metadata (e.g. '$meta' fields)
         // which was held by this PropertyLoadSaver.
         Save(withMeta bool) (PropertyMap, error)
 
-        MetaGetter
-
-        // GetAllMeta returns a PropertyMap with all of the metadata in this
-        // PropertyLoadSaver. If a metadata field has an error during serialization,
-        // it is skipped.
-        GetAllMeta() PropertyMap
-
-        // SetMeta allows you to set the current value of the meta-keyed field.
-        SetMeta(key string, val interface{}) error
+        MetaGetterSetter
 
         // Problem indicates that this PLS has a fatal problem. Usually this is
         // set when the underlying struct has recursion, invalid field types, nested
         // slices, etc.
         Problem() error
 }
 
+// MetaGetterSetter is the subset of PropertyLoadSaver which pertains to
+// getting and saving metadata.
+//
+// A *struct may implement this interface to provide metadata which is
+// supplimental to the variety described by GetPLS. For example, this could be
+// used to implement a parsed-out $kind or $id.
+type MetaGetterSetter interface {
+        MetaGetter
+
+        // GetAllMeta returns a PropertyMap with all of the metadata in this
+        // MetaGetterSetter. If a metadata field has an error during serialization,
+        // it is skipped.
+        //
+        // If a *struct is implementing this, then it only needs to return the
+        // metadata fields which would be returned by its GetMeta implementation, and
+        // the `GetPLS` implementation will add any statically-defined metadata
+        // fields. So if GetMeta provides $id, but there's a simple tagged field for
+        // $kind, this method is only expected to return a PropertyMap with "$id".
+        GetAllMeta() PropertyMap
+
+        // SetMeta allows you to set the current value of the meta-keyed field.
+        SetMeta(key string, val interface{}) error
+}
+
 // PropertyMap represents the contents of a datastore entity in a generic way.
 // It maps from property name to a list of property values which correspond to
 // that property name. It is the spiritual successor to PropertyList from the
 // original SDK.
 //
 // PropertyMap may contain "meta" values, which are keyed with a '$' prefix.
 // Technically the datastore allows arbitrary property names, but all of the
 // SDKs go out of their way to try to make all property names valid programming
 // language tokens. Special values must correspond to a single Property...
 // corresponding to 0 is equivalent to unset, and corresponding to >1 is an
@@ skipping 118 matching lines @@
         dflt = UpconvertUnderlyingType(dflt)
         cur, err := gm(key)
         if err != nil {
                 return dflt
         }
         if dflt != nil && reflect.TypeOf(cur) != reflect.TypeOf(dflt) {
                 return dflt
         }
         return cur
 }