Re-add metadata passthrough on Get operations

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 (
         "errors"
         "fmt"
         "math"
@@ skipping 296 matching lines @@
         }
         p.propType = pt
         p.value = value
         p.indexSetting = is
         if t == typeOfByteSlice {
                 p.indexSetting = NoIndex
         }
         return
 }
 
-// PropertyLoadSaver may be implemented by a user type, and Interface will
-// use this interface to serialize the type instead of trying to automatically
-// create a serialization codec for it with helper.GetPLS.
-type PropertyLoadSaver interface {
-        // Load takes the values from the given map and attempts to save them into
-        // the underlying object (usually a struct or a PropertyMap). If a fatal
-        // 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. '$meta' fields)
-        // which was held by this PropertyLoadSaver.
-        Save(withMeta bool) (PropertyMap, error)
-
+// MetaGetter is a subinterface of PropertyLoadSaver, but is also used to
+// abstract the meta argument for RawInterface.GetMulti.
+type MetaGetter interface {
         // GetMeta will get information about the field which has the struct tag in
         // the form of `gae:"$<key>[,<default>]?"`.
         //
         // Supported metadata types are:
         //   int64  - may have default (ascii encoded base-10)
         //   string - may have default
         //   Toggle - MUST have default ("true" or "false")
         //   Key    - NO default allowed
         //
         // Struct fields of type Toggle (which is an Auto/On/Off) require you to
@@ skipping 12 matching lines @@
         //   // val == 10
         //   // err == nil
         //
         //   type MyStruct struct {
         //     TFlag Toggle `gae:"$flag1,true"`  // defaults to true
         //     FFlag Toggle `gae:"$flag2,false"` // defaults to false
         //     // BadFlag  Toggle `gae:"$flag3"` // ILLEGAL
         //   }
         GetMeta(key string) (interface{}, error)
 
+        // GetMetaDefault is GetMeta, but with a default.
+        //
+        // If the metadata key is not available, or its type doesn't equal the
+        // homogenized type of dflt, then dflt will be returned.
+        //
+        // Type homogenization:
+        //   signed integer types -> int64
+        //   bool                 -> Toggle fields (bool)
+        //
+        // Example:
+        //   pls.GetMetaDefault("foo", 100).(int64)
+        GetMetaDefault(key string, dflt interface{}) interface{}
+}
+
+// PropertyLoadSaver may be implemented by a user type, and Interface will
+// use this interface to serialize the type instead of trying to automatically
+// create a serialization codec for it with helper.GetPLS.
+type PropertyLoadSaver interface {
+        // Load takes the values from the given map and attempts to save them into
+        // the underlying object (usually a struct or a PropertyMap). If a fatal
+        // 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
+
         // SetMeta allows you to set the current value of the meta-keyed field.
         SetMeta(key string, val interface{}) error
 
         // 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
 }
 
 // PropertyMap represents the contents of a datastore entity in a generic way.
@@ skipping 53 matching lines @@
         v, ok := pm["$"+key]
         if !ok || len(v) == 0 {
                 return nil, ErrMetaFieldUnset
         }
         if len(v) > 1 {
                 return nil, errors.New("gae: too many values for Meta key")
         }
         return v[0].Value(), nil
 }
 
+func (pm PropertyMap) GetMetaDefault(key string, dflt interface{}) interface{} {
+        return GetMetaDefaultImpl(pm.GetMeta, key, dflt)
+}
+
 // SetMeta implements PropertyLoadSaver.SetMeta. It will only return an error
 // if `val` has an invalid type (e.g. not one supported by Property).
 func (pm PropertyMap) SetMeta(key string, val interface{}) error {
         prop := Property{}
         if err := prop.SetValue(val, NoIndex); err != nil {
                 return err
         }
         pm["$"+key] = []Property{prop}
         return nil
 }
 
 // Problem implements PropertyLoadSaver.Problem. It ALWAYS returns nil.
 func (pm PropertyMap) Problem() error {
         return nil
 }
+
+// GetMetaDefaultImpl is the implementation of PropertyLoadSaver.GetMetaDefault.
+//
+// It takes the normal GetMeta function, the key and the default, and returns
+// the value according to PropertyLoadSaver.GetMetaDefault.
+func GetMetaDefaultImpl(gm func(string) (interface{}, error), key string, dflt interface{}) interface{} {
+        cur, err := gm(key)
+        if err != nil {
+                return dflt
+        }
+        dflt, typ := UpconvertUnderlyingType(dflt, reflect.TypeOf(dflt))
+        if reflect.TypeOf(cur) != typ {
+                return dflt
+        }
+        return cur
+}