作为java中的O/R Mapping的使用者来说，看到rails中的activeRecord的实现后不禁汗颜，原来O/R Mapping可以如此简单。
我反复看过几遍activeRecord的文档，觉得其灵活之处不亚于hibernate,而且简单实用。
现作笔录如下：
1.关联
n:1: belongs_to: XXXX
1:n: has_many:XXX
1:1: has_one :XXX
m:n: has_and_belongs_to_many:XXX
连接join: :through => :XXX (for ex:has_many :photos :through => :slides) rails 1.1才有此功能
特殊关联：
acts_as_list ：列表
acts_as_tree ：父子关系
acts_as_nested_set

class Slide < ActiveRecord::Base
  belongs_to :slideshow
  acts_as_list :scope => "slideshow_id"
  belongs_to :photo
end
表示sildes表中的slideshow_id为belongs_to :slideshow关系中的外键(FK)
class Category < ActiveRecord::Base
  acts_as_tree
end

2.级联
has_many :XXX, dependent => true

3.设定主键：rails不支持复合主键，rails默认数据表中的主键名称为id,如果您的主键不为id，如下设置：
class Photo < ActiveRecord::Base
  set_primary_key "created_at"
end
4.domain中的继承关系
class Photographer < Person
end

class Person < ActiveRecord::Base
end
5.domain中的组件关系
CREATE TABLE people (
   id INT AUTO_INCREMENT NOT NULL,
   type VARCHAR(20),
   name VARCHAR(20),
   email VARCHAR(30),
   street_address VARCHAR(30),
   city VARCHAR(30),
   state VARCHAR(20),
   zip INTEGER(5),
   camera VARCHAR(20),
   PRIMARY KEY (id)
);

class Person < ActiveRecord::Base
  composed_of :address, :class_name => "Address",
              :mapping => [[:street_address, :street_address],
                           [:city, :city],
                           [:state, :State],
                           [:zip, :zip]]
end
class Address
  def initialize(street_address, city, state, zip)
    @street_address = street_address
    @city = city
    @state = state
    @zip = zip
  end

  attr_reader :street_address, :city, :state, :zip
end
6.查找记录
find_by_<column_name> find_by_id
find_by_<column_name>_and_<column_name> find_by_name_and_email(name, email)
7.验证字段有效性
class Photo < ActiveRecord::Base
  validates_presence_of :filename 验证是否为空
end

validates_format_of :email,
                    :with => /^([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})$/i
validates_length_of :name, :within => 6..100                   
8.事务处理
def transfer(from, to, amount)
  Account.transaction do
    from.debit(amount)
    to.credit(amount)
  end
end