datastore/query.go
func (q *Query) KeysOnly() *Query {
	q = q.clone()
	q.keysOnly = true
	return q
}
func (q *Query) KeysOnly() *Query {
	q = q.clone()
	q.keysOnly = true
	return q
}
// KeysOnly returns a derivative query that yields only keys, not keys and
// entities. It cannot be used with projection queries.