public function getForcedQualificationCustomers()
    {
        if (is_null($this->cacheForcedRanks)) {
            /* get Customer IDs from DB to map ranks to Mage IDs instead of MLM IDs */
            $custIds = $this->getForcedCustomersIds();
            /* get all ranks with configuration parameters for all schemes */
            $ranks = $this->getCfgParamsByRanks();
            $this->cacheForcedRanks = [];
            foreach ($custIds as $item) {
                $custId = $item[Customer::A_CUSTOMER_REF];
                $ref = $item[Customer::A_MLM_ID];
                $rankCode = $this->QUALIFIED_CUSTOMERS[$ref][1];
                $cfgParamsWithSchemes = $ranks[$rankCode];
                $this->cacheForcedRanks[$custId] = $cfgParamsWithSchemes;
            }
            /* compose map from customer IDs for quick search */
            $this->cacheForcedCustomerIds = array_keys($this->cacheForcedRanks);
        }
        return $this->cacheForcedRanks;
    }