I'm looking for a web framework that:

1) really works with Ruby 1.9

2) pure Ruby code (i.e. doesn't depend on C-extensions, Java libraries...)

3) has an active community

bonus points:

4) doesn't follow the "let's extend core classes whenever possible" craze

5) good postgres support

Does such a framework exist or am I asking for too much?

Actually I would use Rails, the latest RC is advertised as Ruby 1.9
compatible but I've immediately hit a couple of errors and I
don't have the time ATM to fix compatibility errors in such a big
codebase and push the changes upstream. What's more, the pure
Ruby postgres driver doesn't work anymore.

Merb fails with syntax errors with Ruby 1.9.

Thanks,
  Stefan