How does Rails PostgreSQL automatically creates user access in tutorials and in Heroku even though not explicitly defined in database.yml

# PostgreSQL. Versions 9.1 and up are supported.
#
# Install the pg driver:
#   gem install pg
# On OS X with Homebrew:
#   gem install pg -- --with-pg-config=/usr/local/bin/pg_config
# On OS X with MacPorts:
#   gem install pg -- --with-pg-config=/opt/local/lib/postgresql84/bin/pg_config
# On Windows:
#   gem install pg
#       Choose the win32 build.
#       Install PostgreSQL and put its /bin directory on your path.
#
# Configure Using Gemfile
# gem 'pg'
#
default: &default
  adapter: postgresql
  encoding: unicode
  # For details on connection pooling, see Rails configuration guide
  # http://guides.rubyonrails.org/configuring.html#database-pooling
  pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>

development:
  <<: *default
  database: shoutr_development

  # The specified database role being used to connect to postgres.
  # To create additional roles in postgres see `$ createuser --help`.
  # When left blank, postgres will use the default role. This is
  # the same name as the operating system user that initialized the database.
  #username: shoutr

  # The password associated with the postgres role (username).
  #password:

  # Connect on a TCP socket. Omitted by default since the client uses a
  # domain socket that doesn't need configuration. Windows does not have
  # domain sockets, so uncomment these lines.
  #host: localhost

  # The TCP port the server listens on. Defaults to 5432.
  # If your server runs on a different port number, change accordingly.
  #port: 5432

  # Schema search path. The server defaults to $user,public
  #schema_search_path: myapp,sharedapp,public

  # Minimum log levels, in increasing order:
  #   debug5, debug4, debug3, debug2, debug1,
  #   log, notice, warning, error, fatal, and panic
  # Defaults to warning.
  #min_messages: notice

# Warning: The database defined as "test" will be erased and
# re-generated from your development database when you run "rake".
# Do not set this db to the same as development or production.
test:
  <<: *default
  database: shoutr_test

# As with config/secrets.yml, you never want to store sensitive information,
# like your database password, in your source code. If your source code is
# ever seen by anyone, they now have access to your database.
#
# Instead, provide the password as a unix environment variable when you boot
# the app. Read http://guides.rubyonrails.org/configuring.html#configuring-a-database
# for a full rundown on how to provide these environment variables in a
# production deployment.
#
# On Heroku and other platform providers, you may have a full connection URL
# available as an environment variable. For example:
#
#   DATABASE_URL="postgres://myuser:mypass@localhost/somedatabase"
#
# You can use this database configuration with:
#
#   production:
#     url: <%= ENV['DATABASE_URL'] %>
#
production:
  <<: *default
  database: shoutr_production
  username: shoutr
  password: <%= ENV['SHOUTR_DATABASE_PASSWORD'] %>

As you can see in the config/database.yml file, the password for both development and production envoirnments are not defined, those users are also not created in the PostgreSQL however i can still push to Heroku and Heroku can somehow still provision a PostgreSQL production environment database which the application utilizes.

I am trying to learn ruby on rails development and this tutorial and from 3:50 mark, the author does bundle install and immediately does db:create successfully implying he can connect to PostgreSQL in development environment without provisioning user and subsequently changing the values in config/database.yml for development env, while if i do the same results in error, which of couse can be solved by creating user in PostgreSQL and changing the credentials in the yml file, however every tutorial i have stumbled upon do not show creating of users:

/home/webmaster/gems/gems/activesupport-5.2.2/lib/active_support/dependencies.rb:291:in `require'
bin/rails:4:in `<main>'

Caused by:
PG::InsufficientPrivilege: ERROR:  permission denied to create database

Solution

There are two main ways to access a local postgresql database. One is using a specific user/password combination in the database.yml file. The other is to set up local postgres to trust your user identity.

For the latter case you need to edit the pg_hba.conf file to have either a trust line (allow anyone to connect, just pass the username) or a peer or ident (depending on the connection - socket or tcp - allows only a postgres role that has the same name as the connecting user, in your case probably webmaster)

For example:

# Allow any user on the local system to connect to any database with
# any database user name using Unix-domain sockets (the default for local
# connections).
#
# TYPE  DATABASE        USER            ADDRESS                 METHOD
local   all             all                                     trust

# The same using local loopback TCP/IP connections.
#
# TYPE  DATABASE        USER            ADDRESS                 METHOD
host    all             all             127.0.0.1/32            trust

You can find more examples here: https://www.postgresql.org/docs/devel/auth-pg-hba-conf.html

WARNING do not use this config on a production server!