获取默认地址，如果没有则取第一条

var addresses []model.Address
if ret := s.db.WithContext(ctx).Model(&model.Address{}).
    Where("member_id = ?", memberId).Find(&addresses); ret.Error != nil {
    return
}
if len(addresses) > 0 {
    defineAddressIndex := 0
    for i, address := range addresses {
        if address.IsDefault == 1 {
            defineAddressIndex = i
            break
        }
    }
    result.Address = addresses[defineAddressIndex]
}