protected function cacheUnsetByRef(string $primary_key, $ref_key)
    {
        $ref_key = (string) $ref_key; // Force string.

        if (empty($this->x___cache['x___refs'][$primary_key][$ref_key])) {
            return; // Nothing to do here.
        }
        foreach (array_keys($this->x___cache['x___refs'][$primary_key][$ref_key]) as $_key) {
            $this->x___cache['x___refs'][$primary_key][$ref_key][$_key] = null;
        } // unset($_key); // Housekeeping.
        unset($this->x___cache['x___refs'][$primary_key][$ref_key]);
    }