function is_addon_connected( $id_or_slug ) {
            $this->_logger->entrance();

            $sites = self::get_all_sites( WP_FS__MODULE_TYPE_PLUGIN );

            $addon_id = self::get_module_id( $id_or_slug );
            $addon    = $this->get_addon( $addon_id );
            $slug     = $addon->slug;
            if ( ! isset( $sites[ $slug ] ) ) {
                return false;
            }

            $site = $sites[ $slug ];

            $plugin = FS_Plugin_Manager::instance( $addon_id )->get();

            if ( $plugin->parent_plugin_id != $this->_plugin->id ) {
                // The given slug do NOT belong to any of the plugin's add-ons.
                return false;
            }

            return ( is_object( $site ) &&
                     is_numeric( $site->id ) &&
                     is_numeric( $site->user_id ) &&
                     FS_Plugin_Plan::is_valid_id( $site->plan_id )
            );
        }