func saveEntity(defaultAppID string, key *Key, src interface{}) (*pb.EntityProto, error) {
	var err error
	var props []Property
	if e, ok := src.(PropertyLoadSaver); ok {
		props, err = e.Save()
	} else {
		props, err = SaveStruct(src)
	}
	if err != nil {
		return nil, err
	}
	return propertiesToProto(defaultAppID, key, props)
}
saveEntity saves an EntityProto into a PropertyLoadSaver or struct pointer.
package datastore
import (
	"errors"
	"fmt"
	"math"
	"reflect"
	"time"
	"github.com/golang/protobuf/proto"
	"google.golang.org/appengine"
	pb "google.golang.org/appengine/internal/datastore"
)
func toUnixMicro(t time.Time) int64 {
	return t.Unix()*1e6 + int64(t.Nanosecond()/1e3)
}
func fromUnixMicro(t int64) time.Time {
	return time.Unix(t/1e6, (t%1e6)*1e3)
}
var (
	minTime = time.Unix(int64(math.MinInt64)/1e6, (int64(math.MinInt64)%1e6)*1e3)
	maxTime = time.Unix(int64(math.MaxInt64)/1e6, (int64(math.MaxInt64)%1e6)*1e3)
)
func valueToProto(defaultAppID, name string, v reflect.Value, multiple bool) (p *pb.Property, errStr string) {
	var (
		pv          pb.PropertyValue
		unsupported bool
	)
	switch v.Kind() {
	case reflect.Invalid:
	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
		pv.Int64Value = proto.Int64(v.Int())
	case reflect.Bool:
		pv.BooleanValue = proto.Bool(v.Bool())
	case reflect.String:
		pv.StringValue = proto.String(v.String())
	case reflect.Float32, reflect.Float64:
		pv.DoubleValue = proto.Float64(v.Float())
	case reflect.Ptr:
		if k, ok := v.Interface().(*Key); ok {
			if k != nil {
				pv.Referencevalue = keyToReferenceValue(defaultAppID, k)
			}
		} else {
			unsupported = true
		}
	case reflect.Struct:
		switch t := v.Interface().(type) {
		case time.Time:
			if t.Before(minTime) || t.After(maxTime) {
				return nil, "time value out of range"
			}
			pv.Int64Value = proto.Int64(toUnixMicro(t))
		case appengine.GeoPoint:
			if !t.Valid() {
				return nil, "invalid GeoPoint value"
			}
			pv.Pointvalue = &pb.PropertyValue_PointValue{X: &t.Lat, Y: &t.Lng}
		default:
			unsupported = true
		}
	case reflect.Slice:
		if b, ok := v.Interface().([]byte); ok {
			pv.StringValue = proto.String(string(b))
		} else {
			unsupported = true
		}
	default:
		unsupported = true
	}
	if unsupported {
		return nil, "unsupported datastore value type: " + v.Type().String()
	}
	p = &pb.Property{
		Name:     proto.String(name),
		Value:    &pv,
		Multiple: proto.Bool(multiple),
	}
	if v.IsValid() {
		switch v.Interface().(type) {
		case []byte:
			p.Meaning = pb.Property_BLOB.Enum()
		case ByteString:
			p.Meaning = pb.Property_BYTESTRING.Enum()
		case appengine.BlobKey:
			p.Meaning = pb.Property_BLOBKEY.Enum()
		case time.Time:
			p.Meaning = pb.Property_GD_WHEN.Enum()
		case appengine.GeoPoint:
			p.Meaning = pb.Property_GEORSS_POINT.Enum()
		}
	}
	return p, ""
}