Gem::LoadError: Specified 'postgresql' for database adapter, but the gem is not loaded. Add `gem 'pg'` to your Gemfile

simply adding gem 'pg' to the gemfile didn't work for me.

This worked for me

gem 'pg', '~> 0.20'

Got this answer from

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

Thanks to Piers C

And yeah, gem 'rails_12factor' helps when it's time view Heroku's logs for error messages.

Add this line to your Gemfile inside the :production group (add one if you don't have it).

group :production do
  gem 'pg'
  gem 'rails_12factor'
end

It's very clear from the error its self that gem pg needs to be added to your Gemfile. You might have added it simply but you need to add gem for your development and production machine specifically because the Heroku app is a production machine for your system and your localhost is Development.

Your Gemfile should look like this:

source 'https://rubygems.org'
ruby '2.0.0'


gem 'rails', '4.0.0'
gem 'bootstrap-sass', '2.3.2.0'
gem 'bcrypt-ruby', '3.0.0'
gem 'faker', '1.1.2'
gem 'will_paginate', '3.0.4'
gem 'bootstrap-will_paginate', '0.0.9'

group :development, :test do
  gem 'sqlite3', '1.3.8'
  gem 'rspec-rails', '2.13.1'

end
group :doc do
  gem 'sdoc', '0.3.20', require: false
end

group :production do
  gem 'pg', '0.15.1'
  gem 'rails_12factor'
end