Rails_ActiveRecord_find方法_拾疑_整理_搜集
简而言之：拼sql语句的方法和细节。
public class method find(*args) in ActiveRecord::Base::self in active_record/base.rb
另外可以看看这个帖子http://www.iteye.com/topic/135499的回复中提到的插件：criteria_query，看着好用：
http://agilewebdevelopment.com/plugins/criteria_query?
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#joins
:joins -
#1 Either an SQL fragment for additional joins like "LEFT JOIN comments ON comments.post_id = id" (rarely needed),
这样的话，joins后面的string字符串会完全跟在select * from model_xxx #{joins 字符串} where ... ...?
这样的话，如果写别名的话，根据拼sql规则，需要下面这样写，在joins处先写别名：
Deal.all(:limit => 1, :joins=>"as d join areas as s on d.city_id = s.id ", :select => "d.*, s.* ", :conditions => ["s.id < 10 and d.id < 100"])
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#2 named associations in the same form used for the :include option, which will perform an INNER JOIN on the associated table(s),
等价于include的用法，也就是说，这样的关联用法，必须是在model中声明了的关联，如belongs_to xxx || include yyy
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#3 or an array containing a mixture of both strings and named associations.
等价于#2，只不过有多个关联时，用数组就可以了
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#4 注意=>If the value is a string, then the records will be returned read-only since they will have attributes that do not correspond to the table's columns. Pass :readonly => false to override.
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#5 和include的区别
看看同样的表结构下的例子，更详细的还是要去看源码或实验：
Deal.all(:limit => 1, :joins=>:concern_deal, :conditions => ["deal_id < 10 and concern_deals.user_id < 1000"])
SELECT `deals`.* FROM `deals` INNER JOIN `concern_deals` ON concern_deals.deal_id = deals.id WHERE (deal_id < 10 and concern_deals.user_id < 1000) LIMIT 1
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Deal.all(:limit => 1, :include=>:concern_deal, :conditions => ["deal_id < 10 and concern_deals.user_id < 1000"])
SELECT DISTINCT `deals`.id FROM `deals` LEFT OUTER JOIN `concern_deals` ON concern_deals.deal_id = deals.id WHERE (deal_id < 10 and concern_deals.user_id < 1000) LIMIT 1
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#from
:from - By default, this is the table name of the class, but can be changed to an alternate table name (or even the name of a database view)
也就是说，可以像hibernate那样，model不一定对应表。
这个用find_by_sql也可以实现，只不过这种用法更好看一些～
例子：
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>> ShopClean.first(:select => "id, shop_name, city_id")
=> #<ShopClean id: 1, shop_name: "上海正善食品有限公司", city_id: 0>
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>> ShopClean.first(:select => "id, shop_name, city_id", :from => "shops")
=> #<ShopClean id: 1, shop_name: "品尚豆捞 ", city_id: 1>
>> ShopClean.find_by_sql("SELECT id, shop_name, city_id FROM shops LIMIT 1")=> [#<ShopClean id: 1, shop_name: "品尚豆捞 ", city_id: 1>]?
#select
:select - By default, this is "*" as in "SELECT * FROM", but can be changed if you, for example, want to do a join but not include the joined columns. Takes a string with the SELECT SQL fragment (e.g. "id, name").
这中用法的拼sql方法就是替换掉默认的SELECT * FROM，所以从拼sql的角度来看，可以在里面写count等语句
例如：
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?> sp = ShopClean.first(:select => "city_id, count(*) cnt", :conditions => ["city_id = 1"], :group => "city_id")
=> #<ShopClean city_id: 1>
>> sp.city_id
=> 1
>> sp.cnt
=> "2584"
#conditions
:conditions - An SQL fragment like "administrator = 1", [ "user_name = ?", username ], or ["user_name = :user_name", { :user_name => user_name }]. See conditions in the intro.
说白了就是拼where语句
注意可以使用hibernate类似的占位符，可以防止sql注入吧～
["user_name = :user_name", { :user_name => user_name }
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