for OS X
brew install postgresql
env ARCHFLAGS="-arch x86_64" gem install postgres
createdb -T template0 -E UTF8 activerecord_unittest
createdb -T template0 -E UTF8 activerecord_unittest2

for ubuntu
#sudo apt-get update
#sudo apt-get upgrade
#sudo apt-get install aptitude
#sudo aptitude install build-essential

sudo aptitude install postgresql postgresql-client libpq-dev ruby1.8-dev
gem install postgres


cloning rails repository and running tests
git clone git://github.com/rails/rails.git

cd rails/activerecord
rake test_postgresql