1、获取数据
获取第一条、最后一条记录
Model.first
Model.first(options)
Model.find(:first, options)
Model.last
Model.last(options)
Model.find(:last, options)
通过id获取记录
Model.find(1, 10, options)
Model.find([1, 10], options)
.find all
对一组数据进行相同操作
User.all.each do |user|
NewsLetter.weekly_deliver(user)
end
如果表记录数比较大，这种方式比较耗资源，因为它会一次载入整个表的数据。改用以下这种方式，它每次只载入1000行，然后逐步yield完整个表
代码如下:
User.find_each do |user|
NewsLetter.weekly_deliver(user)
end
自定义方式，find_each接受和find同样的options
代码如下:
User.find_each(:batch_size => 5000, :start => 2000) do |user|
NewsLetter.weekly_deliver(user)
end
find_in_batches，和find_each相似，但它yield时传递的是model对象数组，而不是单个model对象
代码如下:
Invoice.find_in_batches(:include => :invoice_lines) do |invoices|
export.add_invoices(invoices)
end

2、查询条件
通过替换？来传递条件值，可避免SQL注入
代码如下:
Client.first(:conditions => ["orders_count = ?", params[:orders])
symbol占位条件
代码如下:
Client.all(:conditions => ["created_at >= :start_date AND created_at <= :end_date", {:start_date => params[:start_date], :end_date => params[:end_date] }])
范围条件 in(集合)
代码如下:
Client.all(:conditions => ["created_at IN (?)", (params[:start_date].to_date)..(params[:end_date].to_date])
生成sql
SELECT * FROM users WHERE (created_at IN ('2007-12-31','2008-01-01','2008-01-02','2008-01-03','2008-01-04','2008-01-05', '2008-01-06','2008-01-07','2008-01-08'))
如果要生成日期时间，再加上.to_time
代码如下:params[:start_date].to_date.to_time，生成2007-12-01 00:00:00格式
有上数据库会在以上条件中报错，如Mysql会报查询语句过长的错误，此时可以改成created_at > ? AND created_at < ?的形式
Hash条件
代码如下:
Client.all(:conditions => {:locked => true })
带范围条件
代码如下:
Client.all(:conditons => {:created => (Time.now.midnight - 1.day)..Time.now.midnight})
生成sql
SELECT * FROM clients WHERE (clients.created_at BETWEEN '2008-12-21 00:00:00' AND '2008-12-22 00:00:00')
集合条件
代码如下:
Client.all(:conditons => {:orders_count => [1,3,5])
生成sql
SELECT * FROM clients WHERE (clients.orders_count IN (1,3,5))

3、查询选项
排序
复制代码 代码如下:
#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 代码如下:
Client.all(:select => "viewable_by, locked")
#使用函数
Client.all(:select => "DISTINCT(name)")
限定和偏移Limit and Offset
代码如下:
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
代码如下:
Order.all(:group => "date(created_at)", :order => "created_at")
生成sql
代码如下:SELECT * FROM orders GROUP BY date(created_at)
Having
代码如下:
Order.all(:group => "date(created_at)", :having => ["created_at > ?", 1.month.ago)
生成sql
代码如下:SELECT * FROM orders GROUP BY date(created_at) HAVING created_at > '2009-01-15'
只读
代码如下:
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，复制代码 代码如下:
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
代码如下:
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
代码如下:
Item.transaction do
i = Item.first(:lock => true)
i.name = 'Jones'
i.save
end
Mysql执行返回
复制代码 代码如下: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
代码如下:
Item.transaction do i = Item.find(1, :lock => "LOCK IN SHARE MODE") i.increment!(:views) end

4、关联表
代码如下:
Client.all(:joins => "LEFT OUTER JOIN address ON addresses.client_id = clients.id')
生成sql
代码如下:SELECT clients.* FROM clients LEFT OUTER JOIN addresses ON addresses.client_id = clients.id
使用Array、Hash、Named Associations关联表
有如下model
代码如下:
class Category < ActiveRecord::Base
has_many :posts
end
class Post < ActiveRecord::Base
belongs_to :category
has_many :comments
has_many :tags
end
class Comments <ActiveRecord::Base
belongs_to :post
has_one :guest
end
class Guest < ActiveRecord::Base
belongs_to :comment
end
代码如下:
#关联一个关系
Category.all :joins => :posts
#关联多个关系
Post.all :joins => [:category, :comments]
#嵌套关联
Category.all :joins => {:posts => [{:comments => :guest}, :tags]}
为关联查询结果设定条件复制代码 代码如下:
time_range = (Time.now.midnight - 1.day)..Time.now.midnight Client.all :joins => :orders, :conditions => {'orders.created_at' => time_ran
#或者
time_range = (Time.now.midnight - 1.day)..Time.now.midnight Client.all :joins => :orders, :conditions => {:orders => {:created_at => time_range}}
5、优化载入
以下代码，需要执行1 + 10次sql
代码如下:
clients = Client.all(:limit => 10) clients.each do |client|
puts client.address.postcode
end
优化：
代码如下:
clients = Client.all(:include => :address, :limit => 10)
clients.each do |client|
puts client.address.postcode
end
一次性载入post的所有分类和评论
代码如下:
Post.all :include => [:category, :comments]
载入category为1的所有post和cooment及tag
代码如下:
Category.find 1, :include => {:posts => [{:comments => :guest}, :tags]}

6、动态查询
代码如下:
Client.find_by_name("Ryan")
Client.find_all_by_name("Ryan")
#！方法，没有记录时抛出ActiveRecord::RecordNotFound异常
Client.find_by_name!("Ryan")
#查询多个字段
Client.find_by_name_and_locked("Ryan", true)
#查询不到时就创建并保存
Client.find_or_create_by_name(params[:name])
#查询不到时创建一个实例，但不保存
Client.find_or_initialize_by_name('Ryan')
7、find_by_sql
代码如下:
Client.find_by_sql("SELECT * FROM clients INNER JOIN orders ON clients.id = orders.client_id ORDER clients.created_at desc")
8、select_all
和find_by_sql类似，但不会用model实例化返回记录，你会得到一个hash数组
代码如下:
Client.connection.select_all("SELECT * FROM clients WHERE id = '1'")
9、判断记录是否存在
代码如下:
#通过id来查询
Client.exists?(1)
Client.exists?(1, 2, 3)
#or
Client.exists?([1,2,3])
#通过其他条件来查询
Client.exists?(:conditions => "first_name = 'Ryan'")
#没有参数时，则：表是空的 ? false : true
Client.exists?
10、计算
代码如下:
#求结果集条数
Client.count(:conditons => "first_name = 'Ryan'")
#求某个字段非空白的条数
Client.count(:age)
#平均值
Client.average("orders_count")
#求最小值
Client.minimum("age")
#求最大值
Client.maximum("age")
#求和
Client.sum("orders_count")