Rails: Up and Running, 2nd Edition

Active Record

Automated Mapping

Automatically maps:

• Tables → classes

• Rows → objects (instances of model classes)

• Columns → object attributes

Table to class mapping uses English plurals:

• An Invoice model class maps to an invoices table.

• A Person model class maps to a people table.

• A Country model class maps to a countries table.

• A SecurityLevel model class maps to a security_levels table.

Learn more: http://api.rubyonrails.com/classes/ActiveRecord/Base.html

Associations

Four ways of associating models (see also Figures B-1 and B-2):

Figure B-1. One-to-one and one-to-many relationships

Figure B-2. Many-to-many relationships

has_one has_many belongs_to has_and_belongs_to_many def Order < ActiveRecord::Base has_many :line_items belongs_to :customer # there's a column "customer_id" in the db table end def LineItem < ActiveRecord::Base belongs_to :order # there's a column "order_id" in the db table end def Customer < ActiveRecord::Base has_many :orders has_one :address, :as => addressable end def Address < ActiveRecord::Base belongs_to :addressable, :polymorphic => true end belongs_to :some_model, :class_name => 'MyClass', # specifies other class name :foreign_key => 'my_real_id', # and primary key :conditions => 'column = 0' # only finds when this condition met :polymporphic => boolean # true ...