[chaley]

Apologies for the churn. I looked at the underlying code for the API methods I used and found a significant inefficiency in what I proposed. The underlying reversed dictionary {id:name} was being generated for *every* field item. For big libraries this could be a significant performance hit.

Here is a version that reverses the id map once then uses it to build the output.

def ids_for_field(db, ids_of_books, field_name):
	# First get all the names for the desired books.
	# Use a set to make them unique
	unique_names = set()
	for tup in db.all_field_for(field_name, ids_of_books).values():
		for val in tup:
			unique_names.add(val)
	# reverse the map of ids to names so id_map[name] gives the id
	id_map = {v:k for k,v in db.get_id_map(field_name).items()}
	# Now build the pairs (id, name)
	id_field_pairs = list()
	for name in unique_names:
		id_field_pairs.append((id_map[name], name))
	return id_field_pairs