The option itself is brilliant and has pretty much eliminated all of my slowdown issues in Theater View where my photos make heavy use of expressions for things like calendars, locations, occassions and population, and of course, the ages of people in photos at the time they were taken.
Static data like this only needs to be calculated once.
Thing is, as time moves on and new files are added, it gets tough tracking files that have data replacing expressions, or more explicitly, those that don't.
I currently have a view that lists all media in details view, with a column for every field with that option set.
The first pane here is "Date Tagged". The second pane is a search list containing filters along the lines of "Has Age", "Has Location" and so on and the name of this pane contains the date I last updated the fields, replacing the expression results with the results themselves. I use that date to filter "Date Tagged" from then to now and update the fields for these new files, finally, renaming the search list pane with the current date so that I know where I got up to when I revisit in a month or two.
This is tiresome and rather imprecise. There has to be a better way, surely, but I cannot think of one, nor can I think of the best way that MC might be able to help. My first thought was that if data had replaced the expression, maybe we could switch on an "Identify" flag that would change the colour of that data in a list. While that would help visually, it wouldn't help with filtering, would it?
Can anyone else think of a streamlined way to stay on top of this?