sql.NullStringをjsonにきれいに入れたり出したりするための何か

optionalstring.go

// localtype パッケージ
// アプリケーションローカルの型はここに格納したい
package localtype

import (
	"database/sql"
	"database/sql/driver"
	"encoding/json"
)

// type OptionalString
// sql.NullStringのラッパー
type OptionalString sql.NullString

// func (o *OptionalString) Scan(value interface{}) error
// Implements Scan for settting value from database Row
func (o *OptionalString) Scan(value interface{}) error {
	if value == nil {
		o.String, o.Valid = "", false
		return nil
	} else {
		o.String, o.Valid = string(value.([]byte)), true
		return nil
	}
	return nil
}

// func Convert(s string) OptionalString
// convert OptionalString from simple string
func ConvertOptionalString(s string) *OptionalString {
	var valided = false
	if len(s) > 0 {
		valided = true
	}
	return &OptionalString{
		Valid:  valided,
		String: s,
	}
}

// func (o *OptionalString) Convert() *sql.NullString
// convert from OptionalString to NullString
func (o *OptionalString) Convert() *sql.NullString {
	return &sql.NullString{
		Valid:  o.Valid,
		String: o.String,
	}
}

// func (o *OptionalString) Value() (driver.Value, error)
// Value is needed such as func Scan
func (o *OptionalString) Value() (driver.Value, error) {
	if o.Valid {
		return o.String, nil
	}
	return "", nil
}

// func (o *OptionalString) GetString() string
// return string for safe
// ofcourse, you're ok to access OptionalString.String directly
func (o *OptionalString) SafeString() string {
	if o.Valid {
		return o.String
	}
	return ""
}

// func (o *OptionalString) MarshalJSON() ([]byte, error)
// convert to Json value
func (o *OptionalString) MarshalJSON() ([]byte, error) {
	if o.Valid {
		return json.Marshal(o.String)
	}
	return json.Marshal("")
}

// func (o *OptionalString) UnmarshalJSON(data []byte) error
// convert from string in Json
func (o *OptionalString) UnmarshalJSON(data []byte) error {
	ns := ConvertOptionalString(string(data)).Convert()
	d, err := json.Marshal(ns)
	if err != nil {
		return err
	}
	return json.Unmarshal(d, &sql.NullString{})
}

https://github.com/guregu/null/blob/master/string.go