how to save entity with parent in gorm

I have entity in gorm

type Category struct {
    ID       string    `json:"id"`
    Name     string    `json:"name"`
    ParentID string    `json:"parent_id" gorm:"default:null`
    Parent   *Category `gorm:"foreignKey:ParentID"`
}

but when I want to save

func SaveItem(ctx *fiber.Ctx) error {
    db := database.DbConn
    emptyParent := Category{
        ID: uuid.NullUUID{}.UUID.String(),
    }
    dto := new(Category)
    if err := ctx.BodyParser(dto); err != nil {
        ctx.Status(503).SendString(err.Error())
        return err
    }

    itemDb := new(Category)
    rows := []Category{}
    db.Where("ID = ?", dto.ID).Find(&rows)
    if len(rows) > 0 {
        itemDb = &rows[0]
        itemDb.Name = dto.Name
        rows := []Category{}
        db.Where("ID = ?", dto.ParentID).Find(&rows)
        if len(rows) > 0 {
            parent := rows[0]
            itemDb.Parent = &parent
        } else {
            itemDb.Parent = &emptyParent
        }
        res := db.Save(itemDb)
        err := res.Error
        if err != nil {
            ctx.Status(503).SendString(err.Error())
        }
        return ctx.JSON(&itemDb)
    } else {
        dto.ID = uuid.NewString()
        rows := []Category{}
        db.Where("ID = ?", dto.ParentID).Find(&rows)
        if len(rows) > 0 {
            parent := rows[0]
            dto.Parent = &parent
        } else {
            dto.Parent = &emptyParent
        }
        res := db.Create(&dto)
        err := res.Error
        if err != nil {
            ctx.Status(503).SendString(err.Error())
        }

        return ctx.JSON(&dto)
    }

}

it cause error "insert or update on table "categories" violates foreign key constraint "fk_categories_parent"

question what is wrong ? Who can help me?

in the example below I tried my best. I expect code example.Thank you!