Product group and product list grouped by Magento

I want the products in the product list by catalog to be grouped.

I have 4 custom products and I want to group them into 1 product displayed in the catalog.

To do this, I created an attribute text (associated code) to link each custom product.

Config 1: code001 - assoccode: code001
Config 2: code002 - assoccode: code001
Config 3: code003 - assoccode: code001
Config 4: code004 - assoccode: code001

My problem is that when I have one custom product that has no other option, it does not appear in the directory list, or if it is a simple product, or if I activate the binding category to yes, I got this error: SQLSTATE[42S22]: Column not found: 1054 Unknown column 'assoccode' in 'group statement'

Is there another option for grouping products by attribute in the catalog product list?

and I expanded the product suite as shown below:

class Module_GroupingConfig_Model_Productcollection extends Mage_Catalog_Model_Resource_Product_Collection
{

    protected $_disabled = false;
    protected $_flatEnabled = array();

    public function getFlatHelper()
    {
        return Mage::helper('catalog/product_flat');
    }

    protected function isEnabled()
    {
        return !Mage::app()->getStore()->isAdmin() && !$this->_disabled;
    }

    public function setDisableGroupBy()
    {
        $this->_disabled = true;
    }

    public function isEnabledFlat()
    {
        // Flat Data can be used only on frontend
        if (Mage::app()->getStore()->isAdmin()) {
            return false;
        }
        $storeId = $this->getStoreId();
        if (!isset($this->_flatEnabled[$storeId])) {
            $flatHelper = $this->getFlatHelper();
            $this->_flatEnabled[$storeId] = $flatHelper->isAvailable() && $flatHelper->isBuilt($storeId);
        }
        return $this->_flatEnabled[$storeId];
    }

    protected function _beforeLoad()
    {
        if ($this->isEnabled()) {
            if ($this->isEnabledFlat()) {

                $this->getSelect()->group('assoccode');
                $this->getSelect()->columns(array('assoccode_color_ids' => new Zend_Db_Expr("GROUP_CONCAT(DISTINCT color ORDER BY color_value DESC SEPARATOR ',')")));

            } else {

                $this->joinAttribute('assoccode', 'catalog_product/assoccode', 'entity_id');
                $this->joinAttribute('color', 'catalog_product/color', 'entity_id');

                $this->getSelect()->group('assoccode');
                $this->getSelect()->columns(array('assoccode_color_ids' => new Zend_Db_Expr("GROUP_CONCAT(DISTINCT at_color.value ORDER BY at_color.value DESC SEPARATOR ',')")));

            }

            $this->getSelect()->columns(array('assoccode_count' => new Zend_Db_Expr('COUNT(*)')));
        }
        return parent::_beforeLoad();
    }

    public function getSelectCountSql()
    {
        if ($this->isEnabled()) {
            $this->_renderFilters();

            if ($this->isEnabledFlat()) {
                $countSelect = $this->_getClearSelect()
                    ->columns('COUNT(DISTINCT assoccode) AS cnt')
                    ->resetJoinLeft();
            } else {
                $countSelect = $this->_getClearSelect()
                    ->columns('COUNT(DISTINCT at_assoccode.value) AS cnt')
                    ->resetJoinLeft();
            }

            $countSelect->reset(Zend_Db_Select::GROUP);

            return $countSelect;
        }
        return parent::getSelectCountSql();
    }

}

Any help is greatly appreciated.