datastore/datastore.go
func Put(c context.Context, key *Key, src interface{}) (*Key, error) {
	k, err := PutMulti(c, []*Key{key}, []interface{}{src})
	if err != nil {
		if me, ok := err.(appengine.MultiError); ok {
			return nil, me[0]
		}
		return nil, err
	}
	return k[0], nil
}
func Put(c context.Context, key *Key, src interface{}) (*Key, error) {
	k, err := PutMulti(c, []*Key{key}, []interface{}{src})
	if err != nil {
		if me, ok := err.(appengine.MultiError); ok {
			return nil, me[0]
		}
		return nil, err
	}
	return k[0], nil
}
// Put saves the entity src into the datastore with key k. src must be a struct
// pointer or implement PropertyLoadSaver; if a struct pointer then any
// unexported fields of that struct will be skipped. If k is an incomplete key,
// the returned key will be a unique key generated by the datastore.