I have problem ....PostgreSQLAdapter::PGconn

I have a problem when i am creating a rails project using postgresql...

uninitialized constant ActiveRecord::ConnectionAdapters::PostgreSQLAdapter::PGconn

I have already installed gem postgres-pr. but not start.. I using rails version 2.3.5 Ruby 1.8.7 PostgresSQL 1.10.2. Please help me thanks.