ActionRecord: find 多表关联查询 joins 属性

# simply inner join
Member.find(:first, :joins=>:group)
=> SELECT * FROM members INNER JOIN groups ON groups.id = members.group_id LIMIT 1

# two inner joins
Member.count(:joins=>[:group, :profile])
=> SELECT count(*) AS count_all FROM members
   INNER JOIN groups   ON groups.id = members.group_id
   INNER JOIN profiles ON profiles.member_id = members.id


# symbol and raw SQL
Member.find(:all, :joins=>[:group, "INNER JOIN addresses USING(address_id)"])
=> SELECT * FROM members
   INNER JOIN groups ON groups.id = members.group_id
   INNER JOIN addresses USING(address_id)