Error: Can't get some «class» of {«class», «class», ...}

I'm writing an Applescript for use in iTunes in which at some point I want to se开发者_运维知识库lect any track from a list of tracks, but the way I expected it to work gives an error. Here's the code:

tell application "iTunes"
    set thePlaylist to the first playlist whose name is "Missing track count"
    -- ...
    -- populate a list of strings: albumList
    -- ...
    repeat with albumName in the albumList
        set theAlbum to (the tracks of thePlaylist whose album is albumName)
        display dialog "Found " & (count theAlbum) & " tracks in the album"
        set aTrack to some track of theAlbum -- ERROR OCCURS HERE
    end repeat
end tell

The error I get when I execute the script from within iTunes is:

Can't get some «class cTrk» of {«class cFlT» id 16112 of «class cUsP» id 15982 of «class cSrc» id 65 of application "iTunes", ... etc}

Now, I don't really see why it doesn't work, although I guess it must have something to do with the fact that the items in theAlbum are file tracks from a user playlist from the source from the iTunes application instead of 'just' tracks. Can anyone help me out here?

In this example I use some item instead of some track, which works OK.

tell application "iTunes"
    set thePlaylist to the first playlist
    set x to (the tracks of thePlaylist)
    set aTrack to some item in x
end tell

results in

URL track id 87 of library playlist id 82 of source id 64 of application "iTunes"

Since all the items in your example inherit from track, I don't know why it doesn't work, but it doesn't.

theAlbum is a list, not a playlist, so it doesn't have track elements; it only has items.

The documentation on lists, where it states "You can also refer to indexed list items by class." is incomplete and thus misleading. It seems you can only do this with to built-in classes. From what I can glean, here's why:

Object specifiers (2) are based on key-value coding. A specifier might identify a property (an object attribute or a to-one relationship) or element (a to-many relationship). In the example, we're dealing with elements. To handle elements, the underlying Objective-C class must implement a collection accessor pattern. That is, it must implement at least -<key>, or -countOf<Key> and -objectIn<Key>AtIndex: (it can, of course, implement all of them). The list class does this for a set number of Applescript classes (if you peeked at the ObjC source for the list class, you'd find methods like countOfApplication and -objectInNumberAtIndex:). It could conceivably support arbitrary element object specifiers with an appropriate -doesNotRecognizeSelector: handler, but lists don't appear to have been implemented this way. Since lists don't have -track, -countOfTrack or -objectInTrackAtIndex: handlers, they can't deal with a specifier such as "first track of trackList".