database.yml

# In database.yml...

# First version : Error occurs when "gem 'sqlite3'" is only on the test & development group.
production:
  <<: *default
  database: db/production.sqlite3

# Second version : works well
production:
    <<: *default
    adapter: postgresql
    encoding: unicode
    database: <%= ENV['RDS_DB_NAME'] %>
    username: <%= ENV['RDS_USERNAME'] %>
    password: <%= ENV['RDS_PASSWORD'] %>
    host: <%= ENV['RDS_HOSTNAME'] %>
    port: <%= ENV['RDS_PORT'] %>


# Third version  : works well
production:
    adapter: postgresql
    encoding: unicode
    database: <%= ENV['RDS_DB_NAME'] %>
    username: <%= ENV['RDS_USERNAME'] %>
    password: <%= ENV['RDS_PASSWORD'] %>
    host: <%= ENV['RDS_HOSTNAME'] %>
    port: <%= ENV['RDS_PORT'] %>