datastore/datastore.go
func referenceValueToKey(r *pb.PropertyValue_ReferenceValue) (k *Key, err error) {
	appID := r.GetApp()
	namespace := r.GetNameSpace()
	for _, e := range r.Pathelement {
		k = &Key{
			kind:      e.GetType(),
			stringID:  e.GetName(),
			intID:     e.GetId(),
			parent:    k,
			appID:     appID,
			namespace: namespace,
		}
		if !k.valid() {
			return nil, ErrInvalidKey
		}
	}
	return
}
func referenceValueToKey(r *pb.PropertyValue_ReferenceValue) (k *Key, err error) {
	appID := r.GetApp()
	namespace := r.GetNameSpace()
	for _, e := range r.Pathelement {
		k = &Key{
			kind:      e.GetType(),
			stringID:  e.GetName(),
			intID:     e.GetId(),
			parent:    k,
			appID:     appID,
			namespace: namespace,
		}
		if !k.valid() {
			return nil, ErrInvalidKey
		}
	}
	return
}
// It's unfortunate that the two semantically equivalent concepts pb.Reference
// and pb.PropertyValue_ReferenceValue aren't the same type. For example, the
// two have different protobuf field numbers.
// referenceValueToKey is the same as protoToKey except the input is a
// PropertyValue_ReferenceValue instead of a Reference.