Quote:
Originally Posted by the_Pan
If you meant Library Maintenance→Restore Database, this does not seem to work.
Even if I create the columns by hand before the restore process, they do not contain any data.
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You need a copy of the file metadata.db made before the columns were deleted. That is where the column definitions and the data in the columns are stored.
Library Maintenance→Restore Database almost surely won't work in your case. The problem: calibre backs up information for books in a file metadata.opf stored in each book folder. When you deleted the columns, calibre started making new backups with the columns removed. It writes these backup files at a rate of 1 backup file every 2 seconds. If calibre was running long enough after you deleted the columns then all the data for those columns are gone. There is no way to recover the deleted data short of restoring all the metadata.opf files from before you deleted the columns. But if you can do that then you can restore the metadata.db file as well.