Code review comment for lp:~lifeless/launchpad/bug-759467

On the subclassing - I would have needed to change many things;
 - the ability to check N schemas
 - the policy about which schemas to return (the way we fold things onto INCOMPLETE)
 - add a backdoor to get the unmapped value (for code that cares e.g. the migrator)
 - add a policy for setting to decide when to permit and when to fold

The N schemas thing could be pulled up as I did into the base class; having done that a property neatly solves all the other issues with ~3 lines of code.

As for the name, We do try pretty hard to only query using BugTaskSet.search because of all the preloading complexities, so I'm not to worried. I'll happily land a s/_/db_/ in future if it starts to gross folk out :).

Thanks for the review!