.ymlファイルの「*」や「<<」「<<: *」とは何か？｜database.ymlの&defaultや<<: *defaultの意味を実例で解説

development: &default
  adapter: postgresql
  database: test-app_development

test:
  <<: *default
  database: test-app_test

production:
  <<: *default
  database: test-app_production

development: &default
  adapter: postgresql
  database: test-app_development

  adapter: postgresql
  database: test-app_development

production:
  <<: *default
  database: test-app_production

production:
  adapter: postgresql
  database: test-app_development
  database: test-app_production

&エイリアス 値

プロパティ名: &エイリアス
  値1
  値2
  ,,,,

*エイリアス

a: 
    - &ankor 1
    - *ankor

{ "a": [ 1, 1 ] }

- &x a:1
- *x

[ "a": 1, "a": 1 ]

a:  
    - &x 1
    - 2
b:  
    - 3
    - *x

{ "a": [ 1, 2 ], "b": [ 3, 1 ] }

a: &x
    - 1
    - 2
b: *x

{ "a": [ 1, 2 ], "b": [ 1, 2 ] }

&x
    - 1
    - 2
*x

Error : Unable to parse.
Line : 1  &x

&x :
    - 1
    - 2
*x

Error : Unable to parse.
Line : 4  *x

a: &x
    c: 1
    d: 2
b: *x

{
	"a": {
		"c": 1,
		"d": 2
	},
	"b": {
		"c": 1,
		"d": 2
	}
}

a: &x
    c: 1
    d: 2
b: 
    *x
    e: 3

Error : Unable to parse.
Line : 5  *x

オブジェクト名: &エイリアス
  プロパティ名1: 値1
  プロパティ名2: 値2
  ,,,,

オブジェクト名': 
  <<: *エイリアス
  プロパティ名: 値

オブジェクト名: &エイリアス
  - 値1
  - 値2
  ,,,,

オブジェクト名': 
  <<: *エイリアス
  プロパティ名: 値

a: &x
  b: 1
  c: 2

d: 
  <<: *x
  e: 3

{ "a": { "b": 1, "c": 2 }, "b": { "b": 1, "c": 2, "e": 3 } }

a: &x
    b: 1
    c: 2

d:
    e: 3
    <<: *x
    f: 4

{ "a": { "b": 1, "c": 2 }, "b": { "e": 3, "b": 1, "c": 2, "f": 4 } }

a: &x
  - 1
  - 2

d: 
  <<: *x
  c: 3

{ "a": { "b": 1, "c": 2 }, "b": { "0": 1, "1": 2, "c": 3 }

a: &x
  b: 1
  c: 2

d: 
  *x
  d: 3

Error : Unable to parse.
Line : 6  *x

#コメントアウト
a: &x
  #bを定義
  b: 1
  #cを定義
  c: 2

d: *x

{ "a": { "b": 1, "c": 2 }, "b": { "b": 1, "c": 2 } }

# PostgreSQL. Versions 9.3 and up are supported.
#
# Install the pg driver:
#   gem install pg
# On macOS with Homebrew:
#   gem install pg -- --with-pg-config=/usr/local/bin/pg_config
# On macOS 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
  # https://guides.rubyonrails.org/configuring.html#database-pooling
  pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>

development:
  <<: *default
  database: rails6_docker_alpine_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 running Rails.
  #username: rails6_docker_alpine

  # 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: rails6_docker_alpine_test

# As with config/credentials.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 or a full connection URL as an environment
# variable when you boot the app. For example:
#
#   DATABASE_URL="postgres://myuser:mypass@localhost/somedatabase"
#
# If the connection URL is provided in the special DATABASE_URL environment
# variable, Rails will automatically merge its configuration values on top of
# the values provided in this file. Alternatively, you can specify a connection
# URL environment variable explicitly:
#
#   production:
#     url: <%= ENV['MY_APP_DATABASE_URL'] %>
#
# Read https://guides.rubyonrails.org/configuring.html#configuring-a-database
# for a full overview on how database connection configuration can be specified.
#
production:
  <<: *default
  database: rails6_docker_alpine_production
  username: rails6_docker_alpine
  password: <%= ENV['RAILS6_DOCKER_ALPINE_DATABASE_PASSWORD'] %>

{
    "default": {
        "adapter": "postgresql",
        "encoding": "unicode",
        "pool": "<%= ENV.fetch(\"RAILS_MAX_THREADS\") { 5 } %>"
    },
    "development": {
        "adapter": "postgresql",
        "encoding": "unicode",
        "pool": "<%= ENV.fetch(\"RAILS_MAX_THREADS\") { 5 } %>",
        "database": "rails6_docker_alpine_development"
    },
    "test": {
        "adapter": "postgresql",
        "encoding": "unicode",
        "pool": "<%= ENV.fetch(\"RAILS_MAX_THREADS\") { 5 } %>",
        "database": "rails6_docker_alpine_test"
    },
    "production": {
        "adapter": "postgresql",
        "encoding": "unicode",
        "pool": "<%= ENV.fetch(\"RAILS_MAX_THREADS\") { 5 } %>",
        "database": "rails6_docker_alpine_production",
        "username": "rails6_docker_alpine",
        "password": "<%= ENV['RAILS6_DOCKER_ALPINE_DATABASE_PASSWORD'] %>"
    }
}

default: &default
  adapter: postgresql
  encoding: unicode
  pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>