Heroku doesn't see gem 'pg' - ruby-on-rails

Question

Need help.
When I try to make
user#X220:~/rails_projects/sample_app$ heroku run rake db:migrate
There is a mistake:
Running rake db:migrate on limitless-fjord-69900.... up, run.2816
rake aborted!
Gem::LoadError: Specified 'postgresql' for database adapter, but the gem is not loaded.
Add gem 'pg' to your Gemfile (and ensure its version is at the minimum required by ActiveRecord).
/app/vendor/bundle/ruby/2.2.0/gems/activerecord-4.2.6/lib/active_record/connection_adapters/connection_specification.rb:177:in rescue in spec'
/app/vendor/bundle/ruby/2.2.0/gems/activerecord-4.2.6/lib/active_record/connection_adapters/connection_specification.rb:174:inspec'
/app/vendor/bundle/ruby/2.2.0/gems/activerecord-4.2.6/lib/active_record/connection_handling.rb:50:in establish_connection'
/app/vendor/bundle/ruby/2.2.0/gems/activerecord-4.2.6/lib/active_record/railtie.rb:120:inblock (2 levels) in '
/app/vendor/bundle/ruby/2.2.0/gems/activesupport-4.2.6/lib/active_support/lazy_load_hooks.rb:38:in instance_eval'
/app/vendor/bundle/ruby/2.2.0/gems/activesupport-4.2.6/lib/active_support/lazy_load_hooks.rb:38:inexecute_hook'
/app/vendor/bundle/ruby/2.2.0/gems/activesupport-4.2.6/lib/active_support/lazy_load_hooks.rb:45:in block in run_load_hooks'
/app/vendor/bundle/ruby/2.2.0/gems/activesupport-4.2.6/lib/active_support/lazy_load_hooks.rb:44:ineach'
/app/vendor/bundle/ruby/2.2.0/gems/activesupport-4.2.6/lib/active_support/lazy_load_hooks.rb:44:in run_load_hooks'
/app/vendor/bundle/ruby/2.2.0/gems/activerecord-4.2.6/lib/active_record/base.rb:315:in'
/app/vendor/bundle/ruby/2.2.0/gems/activerecord-4.2.6/lib/active_record/base.rb:26:in <top (required)>'
/app/vendor/bundle/ruby/2.2.0/gems/activerecord-4.2.6/lib/active_record/tasks/mysql_database_tasks.rb:8:in'
.
.
.
Tasks: TOP => db:migrate => db:load_config
(See full trace by running task with --trace)
My Gemfile is:
'source 'https://rubygems.org'
ruby '2.2.0'
gem 'activerecord', '~> 4.2', '>= 4.2.6'
gem 'rails', '4.2.6'
group :development, :test do
gem 'byebug'
gem 'sqlite3', '~> 1.3', '>= 1.3.11'
gem 'rspec-rails', '~> 2.8'
end
group :test do
gem 'selenium-webdriver', '2.35.1'
gem 'capybara', '2.1.0'
end
gem 'sass-rails', '~> 5.0'
gem 'uglifier', '>= 1.3.0'
gem 'coffee-rails', '~> 4.1.0'
gem 'therubyracer', platforms: :ruby
gem 'jquery-rails'
gem 'turbolinks'
gem 'jbuilder', '~> 2.0'
group :doc do
gem 'sdoc', '~> 0.4.0', require: false
end
gem 'unicorn'
group :development do
gem 'web-console', '~> 2.0'
gem 'spring'
end
group :production do
gem 'rails_12factor'
gem 'pg'
end
What's wrong?

Answer 1

Remove or comment gem 'sqlite3' from your Gemfile even if it in development group.
I don't know why but Heroku doesn't run your application when it contains sqlite gem outside production.
If you are use SQLite in development, you have to comment gem 'sqlite' every time when deploying to Heroku and uncomment after deploying.Better way will be use Postgres in development.

Answer 2

As suggested before, this could have to do with your sqlite gem.
If you still want to use sqlite you can set to ignore that group. From Heroku docs:
To specify groups of gems to not to be installed, you can use the BUNDLE_WITHOUT config var.
$ heroku config:set BUNDLE_WITHOUT="development:test"