protected function query_associated_posts( $post_type, $enabled_key, $enabled_value, $list_key ) {

		global $wpdb;

		// See if we have a cached result first.
		$cache = sprintf( 'membership_%1$d_associated_%2$s', $this->get( 'id' ), $post_type );
		$found = null;
		$ids   = wp_cache_get( $cache, '', false, $found );

		// We don't, perform a query.
		if ( ! $found ) {

			$ids = $wpdb->get_col( // phpcs:ignore WordPress.DB.DirectDatabaseQuery.DirectQuery
				$wpdb->prepare(
					"SELECT metas.post_id
				 FROM {$wpdb->postmeta} AS metas
				 JOIN {$wpdb->postmeta} AS metas2 ON metas2.post_id = metas.post_id
				 JOIN {$wpdb->posts} AS posts ON posts.ID = metas.post_id
				 WHERE 1
				   AND posts.post_status = 'publish'
				   AND posts.post_type = %s
				   AND metas2.meta_key = %s
				   AND metas2.meta_value = %s
				   AND metas.meta_key = %s
				   AND metas.meta_value REGEXP %s;",
					$post_type,
					$enabled_key,
					$enabled_value,
					$list_key,
					'a:[0-9][0-9]*:\{(i:[0-9][0-9]*;(i|s:[0-9][0-9]*):"?[0-9][0-9]*"?;)*(i:[0-9][0-9]*;(i|s:[0-9][0-9]*):"?' . $this->get( 'id' ) . '"?;)'
				)
			);

			// Only return ints.
			$ids = array_map( 'absint', $ids );

			// Cache the result.
			wp_cache_set( $cache, $ids );

		}

		return $ids;

	}