外鍵, :joins, :conditions

JOIN的一些說明
SQL JOIN
left join/right join/inner join的用法比較

example:

goals table

id	name
1	abc
2	def

tasks table

id	name	goals_id
1	task1	1
2	task2	1
3	task3	2

 

想用goal name 做查詢時

@tasks = Task.find(:all,
                              :joins => "inner join goals on tasks.goal_id = goals.id",
                              :conditions => "goals.name = 'abc' ",
                              :select=>"tasks.id, tasks.name, task.goals_id")

如果沒有用:select 會find出

id	name	goals_id	id	name
1	task1	1	1	abc
2	task2	1	1	abc

這樣如果直接查＠tasks.id 都是查到後面的 goal的id 都是1

所以要用:select 去取出tasks table的部份

這樣便可以查出

id	name	goals_id
1	task1	1
2	task2	1