Ruby on Rails数据表格关系

一对一关系实例: one-to-one
class Order < ActiveRecord::Base
has_one :paid_order,
    :class_name =>"Order",
    :foreign_key => "order_id",
    :conditions => "paid_on is not null"
在表格上加order_id (表格名单数_id)
class Invoice < ActiveRecord::Base
belongs_to :order

可选参数:class_name, :foreign_key,  :order, :conditions
:dependent => true #删除主表行时同时删除子行
自定义的order用法:
class Customer < ActiveRecord::Base
has_many :orders
has_one :most_recent_order,
    :class_name => ‘Order’,
    :order => ‘created_at DESC’
end

主.从 将被保存
an_invoice = Invoice.new(…)
order.invoice = an_invoice # invoice被保存
从.主 将要手动保存

新增加的方法:
product(force_reload=false)
product=(obj)
build_product(attributes={})
create_product(attributes={})

一对多关系(one-to-many)
class Order < ActiveRecord::Base
has_many :line_items
可选参数除了上面的,还有:exclusively_dependent, :finder_sql,:counter_sql
:exclusively_dependent 在子行没有其它表格瓜葛的情况下使用, 加快处理速度.
:finder_sql的使用实例:
class Order < ActiveRecord::Base
has_many :rails_line_items,
     :class_name => "LineItem",
     :finder_sql => "select l. * from line_items l, products p " +
     " where l.product_id = p.id " +
     " and p.title like ‘%rails%’"
end
order的用法:
class Order < ActiveRecord::Base
has_many :line_items,
   :order =>"quantity, unit_price DESC"
end

主.从 将被保存
an_invoice = Invoice.new(…)
order.invoices <<an_invoice # invoice

class LineItem < ActiveRecord::Base
belongs_to :order
. . .

has_many后可以引用集合:
order = Order.find(123)
total = 0.0
order.line_items.each do |li|
 total += li.quantity * li.unit_price
end

新增加的方法:
orders(force_reload=false)
orders <<order
orders.push(order1, …)
orders.delete(order1, …)
orders.clear
orders.find(options…)
orders.build(attributes={})
orders.create(attributes={})

多对多关系(many-to-many):
class Product < ActiveRecord::Base
has_and_belongs_to_many :categories
. . .
class Category < ActiveRecord::Base
has_and_belongs_to_many :products
. . .

要创建一个中间表格:
categories_products
两表格名字复数形式相联, 排字母前后排序

表格内连关联键
略

预载子表
用:include将子表内容加入内存,提高查询速度, 但损耗内存:
for post in Post.find(:all, :include => [:author, :comments])
 puts "Post: #{post.title}"
 puts "Written by: #{post.author.name}"
 puts "Last comment on: #{post.comments.first.created_on}"
end

自动计算子表行数
 belongs_to加参数:counter_cache => true
 数据库加 子表格名(复数)_count 段, 并加 :default=>0参数.
 然后用 .size可以读取子表行数.
刷新数据读法:product.line_items(:refresh).size