function getFKDefinition($parentClass, $alias) {
        $def= null;
        $parentClass= $this->loadClass($parentClass);
        if ($parentClass && $alias) {
            if ($aggregates= $this->getAggregates($parentClass)) {
                if (isset ($aggregates[$alias])) {
                    $def= $aggregates[$alias];
                    $def['type']= 'aggregate';
                }
            }
            if ($composites= $this->getComposites($parentClass)) {
                if (isset ($composites[$alias])) {
                    $def= $composites[$alias];
                    $def['type']= 'composite';
                }
            }
        }
        if ($def === null) {
            $this->log(xPDO::LOG_LEVEL_ERROR, 'No foreign key definition for parentClass: ' . $parentClass . ' using relation alias: ' . $alias);
        }
        return $def;
    }