[Amarok] aa4c4f5: Enable filtering by labels for the sql collections

[Nikolaj Hald Nielsen]

> Unless I'm very mistaken this approach does not work correctly. Due to
> the m:n relation of tracks to labels this will not work when querying
> for multiple labels, e.g. it will deliver incorrect results when
> querying for "label:A AND label:B" or various other combinations. And
> I should now, given that I had to deal with the bug reports in the A1
> days. In my opionion we will need various subqueries to make it work
> correctly, which was not possible at all with sqlite.

Unfortunately it seems you are right. "OR" seems to work fine, but
"AND" does not. The thing is, if this is not the way to implement it,
then how to do so is beyond me (I found making just this relatively
small change in the SqlQuerymaker quite intimidating). This is in my
opinion a pretty big issue, as that basically leaves one person, you,
who knows how to make this work correctly, and having any parts of the
code dependent on one person is always a bad idea.

> Personally I vote for reverting this feature because it is incomplete
> and doing it properly in a future release instead of releasing it now.
> Other opinions?

Hmm... I'm not sure. At least I can now find all tracks that I
actually tagged with something, even if more advanced queries do not
work. Without this, we might as well remove the label feature
completely as it is basically useless (it is just another comment
field)

> Oh, and why is there no unit test for a new feature in SqlCollection?
> the test infrastructure is all there, so it's really easy to add that.
> Can we start using a TDD approach please?

I need someone to actually show me how to work with the test system.
Last time I tried running a test, I got 5 pages of XML output and gave
up on trying to figure out what that actually meant.

- Nikolaj