Ruby On Rails

database.yml

• & -- means alias.
• << -- means insert some content here.
• • -- means reference a node by its alias.

defaults: &def  # alias def
  ...
test:
  <<: *def  # insert the default (by its alias) node content here.

Server Mode, production, test, development.

rails s -e production / test / development -p 3000

ActiveRecord

An Object/Relational Mapping (ORM) layer supplied with Rails. It closely follows the standard ORM model, which is as follows −

• tables map to classes
• rows map to objects and
• columns map to object attributes