datastore/query.go
func (q *Query) Project(fieldNames ...string) *Query {
	q = q.clone()
	q.projection = append([]string(nil), fieldNames...)
	return q
}
func (q *Query) Project(fieldNames ...string) *Query {
	q = q.clone()
	q.projection = append([]string(nil), fieldNames...)
	return q
}
// Project returns a derivative query that yields only the given fields. It
// cannot be used with KeysOnly.