"postgresql gem is not loaded" error deploying a Ruby on Rails application on Heroku

This worked for me

gem 'pg', '~> 0.20'

Thanks to Piers C

Got this answer from

Heroku and Rails: Gem Load Error with Postgres, however it is Specified in GEMFILE

Add pg in gemfile

 gem 'pg', '~> 0.20'

then bundle update & commit Gemfile & Gemfile.lock to heroku.
simple include like gem 'pg' will not work.

In your Gemfile

group :production do
  gem 'pg'
end

Then run bundle install and try to deploy on Heroku.

If you want to use PostgreSQL on all environments and not only in production (recommended) add the gem outside the :production group and remove other database adapters such as sqlite.

As a side note, you may also want to add the rails_12factor gem as suggested by Heroku.