$replacement .= $direction;
					if ( $m < ( $meta_field_count - 1 ) ) {
						$replacement .= ', ';
					}
				}

				$pieces['orderby'] = $replacement;
			}
			return $pieces;
		}

		/**
		 * Action hook callback. Prevents vehicles from lingering in the Trash after
		 * they've been deleted if a plugin setting dictates such behavior.
		 *
		 * @param  int $post_id
		 * @return void
		 */
		public function maybe_force_delete( $post_id ) {
			// is the post a vehicle?
			if ( INVP::POST_TYPE !== get_post_type( $post_id ) ) {
				return;
			}

			$settings = INVP::settings();
			if ( ! $settings['skip_trash'] ) {
				return;
			}

			// force delete.
			wp_delete_post( $post_id, true );
		}

		/**
		 * Registers all meta fields our custom post type uses to define a vehicle
		 * and its attachments.
		 *
		 * @return void
		 */
		public function register_meta_fields() {
			// Add meta fields to our post type.
			foreach ( INVP::keys_and_types( true ) as $key_arr ) {

				if ( empty( $key_arr['name'] ) ) {
					continue;
				}
				$key = apply_filters( 'invp_prefix_meta_key', $key_arr['name'] );