rails常用数据库查询操作、方法浅析

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)</div>

通过id获取记录

Model.find(1, 10, options)
Model.find([1, 10], options)
.find all</div>

Model.all(options)</div>

对一组数据进行相同操作

User.all.each do |user|
    NewsLetter.weekly_deliver(user)
end</div>
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

User.find_each do |user|
    NewsLetter.weekly_deliver(user)
end</div>
自定义方式，find_each接受和find同样的options

User.find_each(:batch_size => 5000, :start => 2000) do |user|
    NewsLetter.weekly_deliver(user)
end</div>
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

Invoice.find_in_batches(:include => :invoice_lines) do |invoices|
    export.add_invoices(invoices)
end</div>

2、查询条件

通过替换？来传递条件值，可避免SQL注入

Client.first(:conditions => ["orders_count = ?", params[:orders])</div>
symbol占位条件

Client.all(:conditions => ["created_at >= :start_date AND created_at <= :end_date", {:start_date => params[:start_date], :end_date => params[:end_date] }])</div>
范围条件 in(集合)

Client.all(:conditions => ["created_at IN (?)", (params[:start_date].to_date)..(params[:end_date].to_date])</div>
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'))</div>
如果要生成日期时间，再加上.to_time

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

Client.all(:conditions => {:locked => true })</div>
带范围条件

Client.all(:conditons => {:created => (Time.now.midnight - 1.day)..Time.now.midnight})</div>
生成sql

SELECT * FROM clients WHERE (clients.created_at BETWEEN '2008-12-21 00:00:00' AND '2008-12-22 00:00:00')</div>
集合条件

Client.all(:conditons => {:orders_count => [1,3,5])</div>
生成sql

SELECT * FROM clients WHERE (clients.orders_count IN (1,3,5))</div>

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")</div>
返回指定字段

Client.all(:select => "viewable_by, locked")
#使用函数
Client.all(:select => "DISTINCT(name)")</div>
限定和偏移Limit and Offset

Client.all(:limit => 5)
#生成
SELECT * FROM clients LIMIT 5
Client.all(:li