how to add prefix json string in golang

Question

I am newbie. I have a image in my db which is stored as a path to it. In api json I have to output this path and add a prefix in front of it, how can I do that? My function:

func GetAllSliders(c *gin.Context) {
    var sliders models.Slider

    config.DB.Model(models.Slider{}).Where("id=?", 1).Update("image", ("https://spares-dt.ru/" + models.Slider{}.Image)) //i tried this, but it doesnt work

    if err := config.DB.Find(&sliders).Error; err != nil {
        c.JSON(http.StatusInternalServerError, err.Error())
    } else {
        c.JSON(http.StatusOK, gin.H{"data": sliders})
    }
}

Json output i have:

{
    "data": {
        "id": 1,
        "main_text": "123",
        "upper_text": "123",
        "down_text": "123",
        "image": "data/photos/sliders/image.PNG"
    }
}

I want:

{
    "data": {
        "id": 1,
        "main_text": "123",
        "upper_text": "123",
        "down_text": "123",
        "image": "https://spakes-dt.ru/data/photos/sliders/image.PNG"
    }
}

And my struct:

type Slider struct {
    Id        uint   `json:"id" gorm:"primaryKey"`
    MainText  string `json:"main_text"`
    UpperText string `json:"upper_text"`
    DownText  string `json:"down_text"`
    Image     string `json:"image"`
}

Answer 1

Simply change the Image field's value before JSON marshaling it:

if err := config.DB.Find(&sliders).Error; err != nil {
    c.JSON(http.StatusInternalServerError, err.Error())
} else {
    sliders.Image = "https://spares-dt.ru/" + sliders.Image
    c.JSON(http.StatusOK, gin.H{"data": sliders})
}