how to set ruby1.9.1 as default version on debian squeeze

I am using Debian Squeeze as my OS and am trying to develop a Ruby on Rails projects. For that I first installed some ruby packages.

My ruby versions are: ruby1.8, ruby1.9.1

Then I uninstalled ruby1.8 to make ruby1.9.1 my default ruby.

Now if I run:

$ which ruby

it displays nothing.

If I run:

$ which ruby1.9.1
/usr/bin/ruby1.9.1

However:

$ rails new testApp
bash: /usr/bin/rails: /usr/bin/ruby1.8: bad interpreter: No such file or directory

I don't know what to do. Please guide me.