public function addFieldsMapping(\Magento\Customer\Model\ResourceModel\Grid\Collection $collection)
 {
     // depth
     $fieldAlias = self::AS_FLD_CUSTOMER_DEPTH;
     $fieldFullName = self::AS_TBL_CUST . '.' . Customer::ATTR_DEPTH;
     $collection->addFilterToMap($fieldAlias, $fieldFullName);
     // ref (mlm id)
     $fieldAlias = self::AS_FLD_CUSTOMER_REF;
     $fieldFullName = self::AS_TBL_CUST . '.' . Customer::ATTR_HUMAN_REF;
     $collection->addFilterToMap($fieldAlias, $fieldFullName);
     // parent id
     $fieldAlias = self::AS_FLD_PARENT_ID;
     $fieldFullName = self::AS_TBL_CUST . '.' . Customer::ATTR_PARENT_ID;
     $collection->addFilterToMap($fieldAlias, $fieldFullName);
     // parent ref (mlm id)
     $fieldAlias = self::AS_FLD_PARENT_REF;
     $fieldFullName = self::AS_TBL_PARENT_CUST . '.' . Customer::ATTR_HUMAN_REF;
     $collection->addFilterToMap($fieldAlias, $fieldFullName);
 }