heroku rake db:migrate is not working in rails 4 - ruby-on-rails

Question

When I do rake db:migrate, I get the error,below i have attached my databse.yml and my gem file, tried all the stuff on the internet but the error did'nt get resolved!
rake aborted!
LoadError: cannot load such file -- mysql2
development:
adapter: mysql2
encoding: utf8
database: demo_project_development
pool: 5
username: root
password: root
socket: /var/run/mysqld/mysqld.sock
host: localhost
test:
adapter: mysql2
database: demo_project_test
database: db/development.mysql2
username: root
password: root
pool: 5
timeout: 5000
production:
adapter: postgresql
database: demo_project_production
pool: 5
timeout: 5000
Gemfile
source 'https://rubygems.org'
gem 'rails', '4.2.6'
gem 'sass-rails', '~> 5.0'
gem 'uglifier', '>= 1.3.0'
gem 'coffee-rails', '~> 4.1.0'
gem 'jquery-rails'
gem 'turbolinks'
gem 'jbuilder', '~> 2.0'
gem 'sdoc', '~> 0.4.0', group: :doc
group :production do
gem 'pg', '0.20'
gem 'rails_12factor'
end
group :test do
gem 'byebug'
gem 'mysql2', '>= 0.3.13', '< 0.5'
end
gem 'spring'
gem 'devise'
gem "cancan"
gem 'ckeditor', '4.1.3'
gem "nested_form"
gem "paperclip", "~> 5.0.0"
gem 'bootstrap-sass', '~> 3.3.6'
gem 'kaminari'
gem 'ratyrate'
gem 'thinking-sphinx', '~> 3.3.0'
gem 'delayed_job_active_record'
gem 'rails-api'
gem 'active_model_serializers', '~> 0.10.6'
group :development do
gem 'web-console', '~> 2.0'
gem 'mysql2', '>= 0.3.13', '< 0.5'
end
Below is my application.rb
require File.expand_path('../boot', __FILE__)
require 'rails/all'
Bundler.require(*Rails.groups)
module DemoProject
class Application < Rails::Application
config.active_record.raise_in_transactional_callbacks = true
config.active_job.queue_adapter = :delayed_job
config.api_only = false
end
end
config/envoirment.rb
require File.expand_path('../application', __FILE__)
Rails.application.initialize!
boot.rb
ENV['BUNDLE_GEMFILE'] ||= File.expand_path('../../Gemfile', __FILE__)
require 'bundler/setup' # Set up gems listed in the Gemfile.

Answer 1

I was able to reproduce your error in a dummy Rails 4.2.6 app with your Gemfile and database.yml. I couldn't even push the app to Heroku without raising LoadError: cannot load such file -- mysql2.
I fixed the problem by removing the mysql2 gem from the development and test groups and adding it to the default group. My guess is that running bundle install locally adds it as a dependency to Gemfile.lock which is checked into git and pushed to Heroku. Heroku doesn't install the gem and when the app tries to require it a LoadError is raised.
group :test do
gem 'byebug'
end
group :development do
gem 'web-console', '~> 2.0'
end
gem 'mysql2', '>= 0.3.13', '< 0.5'
A couple notes: if you want to add one gem to two or more specific groups, do not specify them separately. This just means there are two places where you're going to have to remember to change your version dependencies. So instead of this
group :test do
gem 'byebug'
gem 'another_gem'
end
group :development do
gem 'web-console', '~> 2.0'
gem 'another_gem'
end
do this
group :test do
gem 'byebug'
end
group :development do
gem 'web-console', '~> 2.0'
end
group :development, :test do
gem 'another_gem'
end
Also, you shouldn't use MySQL locally and PostgreSQL in production, just use Postgres for both. They are not fully compatible, so you are bound to run into confusing issues down the road. Your best solution to this problem would be to remove mysql2 altogether and configure your dev and test databases for Postgres.