Error Please install the postgresql adapter: `gem install activerecord-postgresql-adapter

I'm installing postgresql adapter for Ruby on Rails 1.2.6 and I found this error for create a model, then I installed 2 files, I am working on Ubuntu 8.04 Hardy Heron, I only write:

sudo apt-get install libdbi-ruby1.8 libpgsql-ruby1.8 ruby1.8-dev

I can create a model and database migration.

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