Error in migrating the Postgres database

I want to create a rails application using postgres database.
Update a username and password terp and agile respectively.
When i run the command rake db:create and rake db:migrate
it is giving mi an error..

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

Please tell me how to resolve this error..

Hope u have the postgre adapter installed.

Hello sir,

I have installed

gem to install is ruby-pg

gem to install is postgres

gem to install is postgres-pr

But still getting the same error...
uninitialized constant
ActiveRecord::ConnectionAdapters::PostgreSQLAdapter::PGconn

Is there any configuration needed for the postgreSQLAdapter...
Please tell me how to resolve this error.....

put in config/environment.rb this string:

config.gem "postgres"

or another gem which you use