NOT NULL constraint failed in go orm

I'm trying to execute operation insert with orm in go.

I do insert and not assign value to time type value like the field:

ReplyTime       time.Time `orm:"index"`

it will throw error: NOT NULL constraint failed: topic.reply_time.

So how can I set this value to be nullable or a default value?

type Topic struct {
	Id              int64
	UId             int64
	Title           string
	Content         string `orm:"size(5000)"`
	Attachment      string
	Created         time.Time `orm:"index"`
	Updated         time.Time `orm:"index"`
	Views           int64 `orm:"index"`
	Author          string
	ReplyTime       time.Time `orm:"index"`
	ReplyCount      int64
	ReplyLastUserId int64
}

func AddTopic(title, content string) error {
	o := orm.NewOrm()
	t := time.Now()

	topic := &Topic{Title:title, Content:content, Created:t, Updated:t}
	_, err := o.Insert(topic)
	return err
}

> So how can I set this value to be nullable or a default value?

You can either:

1. Remove the not null constraint from the database and change the type to something that accepts null values. Go includes a few of those in the standard library (e.g. sql.Null*) but doesn't have one for time.Time. Write your own or use something like github.com/guregu/null which adds support for this.
2. Make sure you always set the ReplyTime field before inserting it in the database.

The "best" solution will depend on your application and what this data signifies. Should it be logically possible for ReplyTime to have "no value" (e.g. user never replied)? If so, then use option 1. If it should always have a value, then use option 2.