[chaley]

Originally Posted by ownedbycats

That works. I was just starting out with the two identifiers as a a test case. Eventually I want to encompass more identifiers in descending priority (e.g. if isbn or goodreads doesn't exist, check asin, then google, then url, etc...).

Then you want to use first_non_empty, in which you can list the order by priority. Example:

program:
	i = field('identifiers');
	first_non_empty(
		select(i, 'isbn'),
		select(i, 'goodreads')
# more as desired
	)