Showing template-computed columns as [rating] stars isn't possible?

[kaoruAngel]

The Context (if you must know)

Spoiler:

I've created some custom fields, in order to be able to distinguish between the three different classes of rating judgements that matter.

For my Calibre usage, the standard `rating` field isn't worthwhile to store one's own personal ratings in, since it will also inevitably serve as the intermingled, standard dumping ground for any other rating source; so I keep that as is.

For my personally assigned ratings, I have two concrete rating fields created:

• one for premature ratings, that, at worst, I might even dole out after a mere skimming, or based on the reputation the text already holds personally within my mind (before I've gotten to read it)
• another for final ratings (those that I've assigned only after having read the parts of the text that I was interested in).

For some common simple display and sorting purposes, however, I use a pseudo-field: a simple template program that surfaces either the final rating (if one has been given), or the possibly void premature rating otherwise.

program: first_non_empty( field('#theFinalRatingFieldNameHere'), field('#thePrematureRatingFieldNameHere') )

The Question
Calibre seemingly doesn't try to recognize if the source of a pseudo-field's output has a special display mode already designated; and alternatively, I can't find a way to explicitly specify that this pseudo-field's column is a rating (the column type is either "ratings, shown with stars", which doesn't allow inputting a template but shows as a rating, or it's a "column built from other columns", which does allow inputting a template but doesn't get interpreted as a rating and appropriately shown with rating stars).

Am I missing something?

[kovidgoyal]

i dont think there is UI for it but you could probably insert it into the column definition by hand in the database, admittedly not for the faint of heart

[chaley]

If you want to format a composite column (a computed column) then you must do the formatting yourself in the template. I see three ways to do what you want.

1. Use column icons to show an image of stars based on the value. This would be the way to go if you want half-star images.
2. Use something like the following:
   

   program:
   # the variable 'v' contains the computed rating
   	v = 3.5;
   # verify that the value <= 5
   	v = cmp(v, 5, v, v, 5);
   # Check if the value is a fraction
   	halfstar = contains(v, '\.', 'y', '');
   # Make the value into an integer (floor)
   	v = re(v, '\..*', '');
   # Get the string with the right number of stars
   	r = cmp(v, 0, '', '', substr('★★★★★', 0, v));
   # Add the 'half' if needed
   	test(halfstar, strcat(r, '½'), r)

   The value 3.5 produces ★★★½
3. Use a custom formatter function to compute a string containing whatever you want.