Quote:
Originally Posted by ownedbycats
There's two ways to do this: a composite or using Action Chains to populate the column. The first one means it'll automatically update, but can't be manually tweaked. The latter you'll need to run the chain if you want to update it.
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The first is done using a custom column of type "Column built from other columns" that uses a template to compute the column's value. The value is computed whenever the column is looked at, either by you or by calibre for some reason. As ownedbycats said, you can't change it except by changing the underlying data for that book, or by changing the computation (which changes all books). Note that you can't change the type of an existing column. You would need to create a new column and probably delete the existing one.
For the second you need to ask in the Action Chains plugin thread. What you do depends on how and when you want it updated.