如何使用动态栏

I want to use dynamic column blob data type in mariadb and use in Go project. when fetch data from database with gorm blob field is nil, Why?

type QrController struct {
}

func (controller *QrController) GetQrInfo(context context.Context) {
    qrCode := context.FormValue("qr")
    db, err := gorm.Open("mysql", "root:@/payro_db?charset=utf8&parseTime=True&loc=Local")
if (err != nil) {
        fmt.Println(err.Error())
        return
    }

    defer db.Close()

    var qr model.BrcQr

    if db.First(&qr, "code=?", qrCode).RecordNotFound() {

    }
      context.JSON(qr)
}

result is

{"qr_id":0,"code":"","type":0,"data":null,"created_at":"0001-01-01T00:00:00Z","created_by":null,"status":0}

but the data field must be "{"merchant_id":1}

this is Brc_Qr struct:

package model

import (
    "api/user"
    "database/sql"
    "time"
    "github.com/guregu/null"
)

var (
    _ = time.Second
    _ = sql.LevelDefault
    _ = null.Bool{}
)

type BrcQr struct {
    QrID      int           `gorm:"column:qr_id;primary_key"    json:"qr_id"`
    Code      string        `gorm:"column:code" json:"code"`
    Type      int           `gorm:"column:type" json:"type"`
    Data      []byte        `gorm:"column:data" json:"data"`
    CreatedAt time.Time     `gorm:"column:created_at"   json:"created_at"`
    CreatedBy null.Int      `gorm:"column:created_by"   json:"created_by"`
    Status    int           `gorm:"column:status" json:"status"`
}

// TableName sets the insert table name for this struct type
func (b *BrcQr) TableName() string {
    return "brc_qr"
}