I was thinking the exact same thing. My superficial reaction is that they could embed jruby in their java code and use the real active record model. this would let them reuse the validation logic and all that good stuff.

I'm really curious if they explored this possibility, and if so why they rejected it.

I think there is more to this story than is being told. They say:

"However, we evolved to need something more than Ruby."

Maybe they don't want to be seen as bashing Ruby and thus inciting a pointless language flamewar, but it would be interesting to know what problem or set of problems became the pebble in their shoe that eventually they couldn't stand it anymore.

'Evolved' kind of implies a smooth and natural progression, whereas swapping languages is often more ... abrupt.