[Heroku] At the time of deployment We're sorry, but something went wrong. Solution

I built it in a local environment and deployed it on Heroku, but I got this screen. We're sorry, but something went wrong.

Cause

The development environment (deveropment) was MySQL, but Heroku's standard database is PostgreSQL, but it was still MySQL.

Workaround

Add the following to gem and apply postgreSQL in production environment.

group :production do
  gem 'pg', '>= 0.18', '< 2.0'
end

• Group: production do applies only to the production environment.

Bundle install, but add --without production because it is only in the development environment and not needed in the production environment.

$ bundle install --without production

Then edit database.yml. I think it looks like this by default.

database.yml

production:
  <<: *default
  database: <app_name>_production
  username: <app_name>
  password: <%= ENV['<app_name>_DATABASE_PASSWORD'] %>

Rewrite this like this.

database.yml

production:
  adapter: postgresql
  encoding: unicode
  pool: 5
  database: <app_name>_production
  username: <app_name>
  password: <%= ENV['<app_name>_DATABASE_PASSWORD'] %>

After that, do heroku push and don't forget to do heroku run rails db: migrate``` and it's OK!