In Rails, I solved the problem where the rake db:migrate command couldn’t execute due to an error, so here’s a memo.

$ rake db:migrate
rake aborted!
no such file to load -- pg

Tasks: TOP => db:migrate
(See full trace by running task with --trace)

The Gemfile in the application root directory had the default SQLite3 configuration, so changing this to use PostgreSQL resolves the issue.

$ vim Gemfile

■ Excerpt from Gemfile

#Comment out the sqlite3 requirement
#gem 'sqlite3-ruby', :require => 'sqlite3'
#Add a pg requirement
gem 'pg', :require => 'pg'

That’s all.

【Reference】

・Ruby/Rails 3 and PostgreSQL | Peter Mac And Associates

・RailsからPostgreSQLに繋がらない～ - Stellaqua - ＴＯＭの技術日記 (Can’t connect to PostgreSQL from Rails - Stellaqua - TOM’s Tech Diary)

That’s all from the Gemba.