How to control the precision and scale?

[yaniv preiss]

I'm using 5 stars and cache cloumn.
I'd like the tooltip to show something like 4.23, but is shows the entire number, e.g. 4.23877363432

The average is a decimal with precision 3 and scale 2
Rails 3.2.8
sqlite and postgre (on heroku)

How do I control this?
Isn't the 'to_nearest' supposed to control that?

Thanks