Decouple PLS from MGS.

service/datastore/interface.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 (
         "golang.org/x/net/context"
 )
 
@@ skipping 45 matching lines @@
         // NewKeyToks constructs a new key in the current appID/Namespace, using the
         // specified key tokens.
         NewKeyToks([]KeyTok) *Key
 
         // KeyForObjErr extracts a key from src.
         //
         // src must be one of:
         //   - *S where S is a struct
         //   - a PropertyLoadSaver
         //
-        // It is expected that the struct or PropertyLoadSaver exposes the
-        // following metadata (as retrieved by PropertyLoadSaver.GetMeta):
+        // It is expected that the struct exposes the following metadata (as retrieved
+        // by MetaGetter.GetMeta):
         //   - "key" (type: Key) - The full datastore key to use. Must not be nil.
         //     OR
         //   - "id" (type: int64 or string) - The id of the Key to create
         //   - "kind" (optional, type: string) - The kind of the Key to create. If
         //     blank or not present, KeyForObjErr will extract the name of the src
         //     object's type.
         //   - "parent" (optional, type: Key) - The parent key to use.
         //
+        // By default, the metadata will be extracted from the struct and its tagged
+        // properties. However, if the struct implements MetaGetterSetter it is
+        // wholly responsible for exporting the required fields. A struct that
+        // implements GetMeta to make some minor tweaks can evoke the defualt behavior
+        // by using GetPLS(s).GetMeta.
+        //
         // If a required metadata item is missing or of the wrong type, then this will
         // return an error.
         KeyForObjErr(src interface{}) (*Key, error)
 
         // RunInTransaction runs f inside of a transaction. See the appengine SDK's
         // documentation for full details on the behavior of transactions in the
         // datastore.
         //
         // Note that the behavior of transactions may change depending on what filters
         // have been installed. It's possible that we'll end up implementing things
@@ skipping 91 matching lines @@
 
         // Testable returns the Testable interface for the implementation, or nil if
         // there is none.
         Testable() Testable
 
         // Raw returns the underlying RawInterface. The Interface and RawInterface may
         // be used interchangably; there's no danger of interleaving access to the
         // datastore via the two.
         Raw() RawInterface
 }