field.go
func (ts *TimeStamp) BeforeInsert() error {
	n := now()
	ts.CreatedAt = n
	ts.UpdatedAt = n
	return nil
}
func (ts *TimeStamp) BeforeInsert() error {
	n := now()
	ts.CreatedAt = n
	ts.UpdatedAt = n
	return nil
}
// BeforeInsert sets current time to CreatedAt and UpdatedAt field.
// It always returns nil.