3、查询选项
排序
#单个排序
Client.all(:order => "created_at ASC")
#多个排序
Client.all(:order => "orders_count ASC, created_at DESC")
返回指定字段
Client.all(:select => "viewable_by, locked")
#使用函数
Client.all(:select => "DISTINCT(name)")
限定和偏移Limit and Offset
Client.all(:limit => 5)
#生成
SELECT * FROM clients LIMIT 5
Client.all(:limit => 5, :offset => 5)
#生成
SELECT * FROM clients LIMIT 5, 5
Group分组
Order.all(:group => "date(created_at)", :order => "created_at")
生成sql
SELECT * FROM orders GROUP BY date(created_at)
Having
Order.all(:group => "date(created_at)", :having => ["created_at > ?", 1.month.ago)
生成sql
SELECT * FROM orders GROUP BY date(created_at) HAVING created_at > '2009-01-15'
只读
client = Client.first(:readonly => true)
client.locked = false
client.save
#对只读对象进行保存将会触发ActiveRecord::ReadOnlyRecord异常
更新时锁定记录
乐观锁Optimistic Locking
为使用乐观锁，须在表里建一个lock_version的字段，每次更新记录时，ActiveRecord自动递增lock_version的值，
c1 = Client.find(1) c2 = Client.find(1) c1.name = "Michael" c1.save c2.name = "should fail" c2.save # Raises a ActiveRecord::StaleObjectError
备注：You must ensure that your database schema defaults the lock_version column to 0.
This behavior can be turned off by setting ActiveRecord::Base.lock_optimistically = false.
指定乐观锁字段名
class Client < ActiveRecord::Base set_locking_column :lock_client_column end
悲观锁Pessimistic Locking
悲观锁定由数据库直接提供
Item.transaction do
i = Item.first(:lock => true)
i.name = 'Jones'
i.save
end
Mysql执行返回
SQL (0.2ms) BEGIN Item Load (0.3ms) SELECT * FROM `items` LIMIT 1 FOR UPDATE Item Update (0.4ms) UPDATE `items` SET `updated_at` = '2009-02-07 18:05:56', `name` = 'Jones' WHERE `id` = 1 SQL (0.8ms) COMMIT
为特定数据库加入原始的lock声明
为Mysql的锁定声明为共享模式，即锁定时仍然可读
Item.transaction do i = Item.find(1, :lock => "LOCK IN SHARE MODE") i.increment!(:views) end
4、关联表
Client.all(:joins => "LEFT OUTER JOIN address ON addresses.client_id = clients.id')
生成sql
SELECT clients.* FROM clients LEFT OUTER JOIN addresses ON addresses.client_id = clients.id
使用Array、Hash、Named Associations关联表
