Add 'user friendly' datastore API.

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"
 )
 
-// GetPLS resolves o into a PropertyLoadSaver. o must be a pointer to a
-// struct of some sort.
-func GetPLS(o interface{}) PropertyLoadSaver {
-        v := reflect.ValueOf(o)
+// GetPLS resolves obj into a PropertyLoadSaver.
+//
+// obj must be a non-nil pointer to a struct of some sort.
+//
+// By default, exported fields will be serialized to/from the datastore. If the
+// field is not exported, it will be skipped by the serialization routines.
+//
+// If a field is of a non-supported type (see Property for the list of supported
+// property types), the resulting PropertyLoadSaver will have a non-nil
+// Problem(). Other problems include duplicate field names (due to tagging),
+// recursively defined structs, nested structures with multiple slices (e.g.
+// slices of slices, either directly `[][]type` or indirectly `[]Embedded` where
+// Embedded contains a slice.)
+//
+// GetPLS supports the following struct tag syntax:
+//   `gae:"fieldName[,noindex]"` -- an alternate fieldname for an exportable
+//          field.  When the struct is serialized or deserialized, fieldName will be
+//      associated with the struct field instead of the field's Go name. This is
+//      useful when writing Go code which interfaces with appengine code written
+//      in other languages (like python) which use lowercase as their default
+//                datastore field names.
+//
+//      A fieldName of "-" means that gae will ignore the field for all
+//      serialization/deserialization.
+//
+//      if noindex is specified, then this field will not be indexed in the
+//      datastore, even if it was an otherwise indexable type. If fieldName is
+//      blank, and noindex is specifed, then fieldName will default to the
+//      field's actual name. Note that by default, all fields (with indexable
+//      types) are indexed.
+//
+//   `gae:"$metaKey[,<value>]` -- indicates a field is metadata. Metadata
+//      can be used to control filter behavior, or to store key data when using
+//      the Interface.KeyForObj* methods. The supported field types are:
+//        - Key
+//        - int64
+//        - string
+//        - Toggle (GetMeta and SetMeta treat the field as if it were bool)
+//      Additionally, int64, string and Toggle allow setting a default value
+//      in the struct field tag (the "<value>" portion).
+//
+//      Only exported fields allow SetMeta, but all fields of appropriate type
+//      allow tagged defaults. See Examples.
+//
+// Example "special" structure. This is supposed to be some sort of datastore
+// singleton object.
+//   struct secretFoo {
+//     // _id and _kind are not exported, so setting their values will not be
+//     // reflected by GetMeta.
+//     _id   int64  `gae:"$id,1"`
+//     _kind string `gae:"$kind,InternalFooSingleton"`
+//
+//     // Value is exported, so can be read and written by the PropertyLoadSaver,
+//     // but secretFoo is shared with a python appengine module which has
+//     // stored this field as 'value' instead of 'Value'.
+//     Value int64  `gae:"value"`
+//   }
+//
+// Example "normal" structure that you might use in a go-only appengine app.
+//   struct User {
+//     ID string `gae:"$id"`
+//     // "kind" is automatically implied by the struct name: "User"
+//     // "parent" is nil... Users are root entities
+//
+//     // 'Name' will serialized to the datastore in the field 'Name'
+//     Name string
+//   }
+//
+//   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
+//   }
+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()
-        t := v.Type()
+        c := getCodec(v.Type())
+        return &structPLS{v, c}
+}
 
+func getCodec(structType reflect.Type) *structCodec {
         structCodecsMutex.RLock()
-        if c, ok := structCodecs[t]; ok {
-                structCodecsMutex.RUnlock()
-                return &structPLS{v, c}
+        c, ok := structCodecs[structType]
+        structCodecsMutex.RUnlock()
+        if ok {
+                return c
         }
-        structCodecsMutex.RUnlock()
 
         structCodecsMutex.Lock()
         defer structCodecsMutex.Unlock()
-        return &structPLS{v, getStructCodecLocked(t)}
+        return getStructCodecLocked(structType)
 }